MultiIndex.get_loc misbehaves on NaNs
#### Code Sample, a copy-pastable example if possible

```python
In [2]: mi = pd.MultiIndex(levels=[[1, 2, 3, 5], [4, 6]], labels=[[3, 1, 2, 0], [1, -1, 0, -1]])

In [3]: mi.get_loc((2, np.nan))
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-d77b20d4d7a2> in <module>()
----> 1 mi.get_loc((2, np.nan))

/home/pietro/nobackup/repo/pandas/pandas/core/indexes/multi.py in get_loc(self, key, method)
   2119             key = _values_from_object(key)
   2120             key = tuple(map(_maybe_str_to_time_stamp, key, self.levels))
-> 2121             return self._engine.get_loc(key)
   2122 
   2123         # -- partial selection or non-unique index

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.MultiIndexObjectEngine.get_loc (pandas/_libs/index.c:14965)()
    616         return super(MultiIndexObjectEngine, self).get_indexer(values)
    617 
--> 618     cpdef get_loc(self, object val):
    619 
    620         # convert a MI to an ndarray

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.MultiIndexObjectEngine.get_loc (pandas/_libs/index.c:14886)()
    621         if hasattr(val, 'values'):
    622             val = val.values
--> 623         return super(MultiIndexObjectEngine, self).get_loc(val)
    624 
    625 

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5832)()
    137             util.set_value_at(arr, loc, value)
    138 
--> 139     cpdef get_loc(self, object val):
    140         if is_definitely_invalid_key(val):
    141             raise TypeError("'{val}' is an invalid key".format(val=val))

/home/pietro/nobackup/repo/pandas/pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5678)()
    159 
    160         try:
--> 161             return self.mapping.get_item(val)
    162         except (TypeError, ValueError):
    163             raise KeyError(val)

/home/pietro/nobackup/repo/pandas/pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:21018)()
   1263                                        sizeof(uint32_t)) # flags
   1264 
-> 1265     cpdef get_item(self, object val):
   1266         cdef khiter_t k
   1267         if val != val or val is None:

/home/pietro/nobackup/repo/pandas/pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:20972)()
   1271             return self.table.vals[k]
   1272         else:
-> 1273             raise KeyError(val)
   1274 
   1275     cpdef set_item(self, object key, Py_ssize_t val):

KeyError: (2, nan)

In [4]: mi.get_indexer(mi.copy())
Out[4]: array([ 0, -1,  2, -1])

In [5]: mi == mi.copy()
Out[5]: array([ True, False,  True, False], dtype=bool)

```
#### Problem description

I think _this_ is actually the cause for [this example](https://github.com/pandas-dev/pandas/issues/18455#issuecomment-346700652), which is different from the one reported at the top of #18455 .

#### Expected Output

``array([ 0, 1,  2, 3])``

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

<details>

INSTALLED VERSIONS
------------------
commit: b45325e283b16ec8869aaea407de8256fc234f33
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.22.0.dev0+201.gb45325e28
pytest: 3.0.6
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.18.1
pyarrow: None
xarray: None
IPython: 5.2.2
sphinx: None
patsy: 0.4.1+dev
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: None
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
