Exception thrown on groupby() when non-grouping field has time type
This code generates an error in Pandas 0.17.0, but works fine in pandas 0.16.2

``` python
import datetime
import pandas as pd
import numpy as np

print pd.show_versions()

SHOW_BUG=True
if SHOW_BUG:
    df = pd.DataFrame({
            'a': 1 * np.ones(10),
            'b': [datetime.datetime.now() for nn in range(10)],
        })
else:
    df = pd.DataFrame({
            'a': 1 * np.ones(10),
            'b': range(10),
        })


def _compute_length(batch):
    return pd.Series({'c': 2})

dfg = df.groupby(by=['a']).apply(_compute_length)
```

Here is the version information:

```
print pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8

pandas: 0.17.0
nose: 1.3.7
pip: 7.1.2
setuptools: 18.4
Cython: 0.22.1
numpy: 1.10.1
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 3.2.0
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.4.1
pytz: 2015.6
blosc: None
bottleneck: 1.0.0
tables: 3.2.0
numexpr: 2.4.3
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 1.0.0
xlsxwriter: 0.7.3
lxml: 3.4.4
bs4: 4.3.2
html5lib: None
httplib2: 0.9.1
apiclient: 1.4.1
sqlalchemy: 1.0.5
pymysql: None
psycopg2: None
None
```

And here is the stack trace

```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-3-072dccab0584> in <module>()
     21     return pd.Series({'c': 2})
     22 
---> 23 dfg = df.groupby(by=['a']).apply(_compute_length)

/Users/rob/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py in apply(self, func, *args, **kwargs)
    711         # ignore SettingWithCopy here in case the user mutates
    712         with option_context('mode.chained_assignment',None):
--> 713             return self._python_apply_general(f)
    714 
    715     def _python_apply_general(self, f):

/Users/rob/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py in _python_apply_general(self, f)
    718 
    719         return self._wrap_applied_output(keys, values,
--> 720                                          not_indexed_same=mutated)
    721 
    722     def aggregate(self, func, *args, **kwargs):

/Users/rob/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py in _wrap_applied_output(self, keys, values, not_indexed_same)
   3125                     date_cols = self._selected_obj.select_dtypes(
   3126                         include=list(_DATELIKE_DTYPES)).columns
-> 3127                     result[date_cols] = (result[date_cols]
   3128                                          ._convert(datetime=True,
   3129                                                           coerce=True))

/Users/rob/anaconda/lib/python2.7/site-packages/pandas/core/frame.py in __getitem__(self, key)
   1906         if isinstance(key, (Series, np.ndarray, Index, list)):
   1907             # either boolean or fancy integer index
-> 1908             return self._getitem_array(key)
   1909         elif isinstance(key, DataFrame):
   1910             return self._getitem_frame(key)

/Users/rob/anaconda/lib/python2.7/site-packages/pandas/core/frame.py in _getitem_array(self, key)
   1950             return self.take(indexer, axis=0, convert=False)
   1951         else:
-> 1952             indexer = self.ix._convert_to_indexer(key, axis=1)
   1953             return self.take(indexer, axis=1, convert=True)
   1954 

/Users/rob/anaconda/lib/python2.7/site-packages/pandas/core/indexing.py in _convert_to_indexer(self, obj, axis, is_setter)
   1119                 mask = check == -1
   1120                 if mask.any():
-> 1121                     raise KeyError('%s not in index' % objarr[mask])
   1122 
   1123                 return _values_from_object(indexer)

KeyError: "['b'] not in index"
```
