group_keys=False stopped working with groupby().resample() in v0.18.0
Updating from 0.17.1 to 0.18.0 breaks `group_keys=False`.

```
>>> data = pd.DataFrame([1, 2, 3, 4], index=pd.date_range("20000101", periods=4))
>>> data.groupby(data.index, group_keys=False).resample('4H', how='last')
```

0.17.1:

```
            0
2000-01-01  1
2000-01-02  2
2000-01-03  3
2000-01-04  4
```

0.18.0:

```
                       0
2000-01-01 2000-01-01  1
2000-01-02 2000-01-02  2
2000-01-03 2000-01-03  3
2000-01-04 2000-01-04  4
```

I also tried `.resample('4H').last()`, and it fails as well. With `sum()` in place of `resample()`, it works correctly.

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-79-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: None
pip: 8.0.2
setuptools: 19.6.2
Cython: None
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: None
sphinx: None
patsy: 0.4.0
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None

```
