Partial date indexing only works with single key
#### Code Sample, a copy-pastable example if possible

```python
In [2]: index = pd.date_range('2001-01-01', periods=100)                                                                                                                                                                                      

In [3]: index.get_loc('2001-01')                                                                                                                                                                                                              
Out[3]: slice(0, 31, None)

In [4]: index.get_indexer(['2001-01'])                                                                                                                                                                                                        
Out[4]: array([-1])

In [5]: s = pd.Series(1, index=index)                                                                                                                                                                                                         

In [6]: s.loc['2001-01']                                                                                                                                                                                                                      
Out[6]: 
2001-01-01    1
2001-01-02    1
2001-01-03    1
2001-01-04    1
2001-01-05    1
2001-01-06    1
2001-01-07    1
2001-01-08    1
2001-01-09    1
2001-01-10    1
2001-01-11    1
2001-01-12    1
2001-01-13    1
2001-01-14    1
2001-01-15    1
2001-01-16    1
2001-01-17    1
2001-01-18    1
2001-01-19    1
2001-01-20    1
2001-01-21    1
2001-01-22    1
2001-01-23    1
2001-01-24    1
2001-01-25    1
2001-01-26    1
2001-01-27    1
2001-01-28    1
2001-01-29    1
2001-01-30    1
2001-01-31    1
Freq: D, dtype: int64

In [7]: s.loc[['2001-01']]                                                                                                                                                                                                                    
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-7-f0d6dc35665a> in <module>
----> 1 s.loc[['2001-01']]

/home/nobackup/repo/pandas/pandas/core/indexing.py in __getitem__(self, key)
   1427 
   1428             maybe_callable = com.apply_if_callable(key, self.obj)
-> 1429             return self._getitem_axis(maybe_callable, axis=axis)
   1430 
   1431     def _is_scalar_access(self, key):

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_axis(self, key, axis)
   1829                     raise ValueError('Cannot index with multidimensional key')
   1830 
-> 1831                 return self._getitem_iterable(key, axis=axis)
   1832 
   1833             # nested tuple slicing

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_iterable(self, key, axis)
   1131             # A collection of keys
   1132             keyarr, indexer = self._get_listlike_indexer(key, axis,
-> 1133                                                          raise_missing=False)
   1134             return self.obj._reindex_with_indexers({axis: [keyarr, indexer]},
   1135                                                    copy=True, allow_dups=True)

/home/nobackup/repo/pandas/pandas/core/indexing.py in _get_listlike_indexer(self, key, axis, raise_missing)
   1087         self._validate_read_indexer(keyarr, indexer,
   1088                                     o._get_axis_number(axis),
-> 1089                                     raise_missing=raise_missing)
   1090         return keyarr, indexer
   1091 

/home/nobackup/repo/pandas/pandas/core/indexing.py in _validate_read_indexer(self, key, indexer, axis, raise_missing)
   1172                 raise KeyError(
   1173                     "None of [{key}] are in the [{axis}]".format(
-> 1174                         key=key, axis=self.obj._get_axis_name(axis)))
   1175 
   1176             # We (temporarily) allow for some missing keys with .loc, except in

KeyError: "None of [Index(['2001-01'], dtype='object')] are in the [index]"


```
#### Problem description

If an indexing mechanism works for individual key, it would be expected to also work for lists of keys, and this is more or less the status quo for other indexes that allow some partial/smart selection.

``IntervalIndex`` works just fine:
```
In [2]: iidx = pd.IntervalIndex.from_breaks([0, 1, 4.6, 7])                                                                                                                                                                                   

In [3]: iidx.get_loc(5)                                                                                                                                                                                                                       
Out[3]: 2

In [4]: iidx.get_indexer([5])                                                                                                                                                                                                                 
Out[4]: array([2])
```
``MultiIndex`` doesn't (will open a bug now)
```
In [5]: midx = pd.MultiIndex.from_product([[0, 1], [2, 3]])                                                                                                                                                                                   

In [6]: midx.get_loc(0)                                                                                                                                                                                                                       
Out[6]: slice(0, 2, None)

In [7]: midx.get_indexer([0])                                                                                                                                                                                                                 
Out[7]: array([-1])
```
... but this is worked around in indexing code (which shouldn't happen):
```
In [8]: pd.Series(index=midx).loc[[0]]                                                                                                                                                                                                        
Out[8]: 
0  2   NaN
   3   NaN
dtype: float64
```

#### Expected Output

The same as when passing a single key.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 7ceefb3f2e1c1f6b0e4435ab4d10ffa4aef4ec36
python           : 3.7.3.candidate.1
python-bits      : 64
OS               : Linux
OS-release       : 4.9.0-9-amd64
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : it_IT.UTF-8
LOCALE           : it_IT.UTF-8

pandas           : 0.25.0.dev0+861.g7ceefb3f2.dirty
numpy            : 1.16.4
pytz             : 2016.7
dateutil         : 2.8.0
pip              : 9.0.1
setuptools       : 41.0.1
Cython           : 0.29.2
pytest           : 4.6.3
hypothesis       : 3.71.11
sphinx           : 1.4.9
blosc            : None
feather          : None
xlsxwriter       : 0.9.6
lxml.etree       : 4.3.2
html5lib         : 0.999999999
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.5.0
pandas_datareader: 0.2.1
bs4              : 4.5.3
bottleneck       : 1.2.1
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.2
matplotlib       : 3.0.2
numexpr          : 2.6.9
openpyxl         : 2.3.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.1.0
sqlalchemy       : 1.0.15
tables           : 3.4.4
xarray           : None
xlrd             : 1.0.0
xlwt             : 1.3.0
xlsxwriter       : 0.9.6


</details>
