incorrect result for 'fast path' .nunique on groupby
#### Code Sample
```python
import pandas as pd


freq = '1D'
# this fails
N_days = 360
# this works
# N_days = 30
N_choices = 30
N_rows = 1000
seed = 0


random_state = pd.np.random.RandomState(seed)


df = pd.DataFrame(dict(
    k=random_state.randint(N_choices, size=N_rows),
    t=pd.Series(pd.DatetimeIndex(['2015-01-01']*N_rows)) +
      pd.Series([pd.DateOffset(el)  # noqa
                 for el in random_state.randint(N_days, size=N_rows)])
    ))
gb = df.groupby(pd.Grouper(key='t', freq=freq))
left = gb.k.nunique()
right = gb.apply(lambda df: df.k.nunique())
# this shows that gb.k.nunique() gives the wrong answer
print(df[df.t == df.t.min()])
print(left.head())
print(right.head())
assert left.equals(right)
```
#### Problem description

These answers are not the same but they should be.  `gb.k.nunique` gives an incorrect answer.

#### Expected Output

expect `assert left.equals(right)` to not fail

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

<details>
In [1]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-56-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: None
</details>
