BUG: groupby, as_index=False still returning group variable as index
#### Code Sample, a copy-pastable example if possible

```
a = pd.DataFrame({'a':[1,1,2,2], 'b':[1,1,2,2], 'c':[1,1,1,1]})
a.groupby(['a','b'], as_index=False).apply(lambda x: 1)

Out[4]: 
a  b
1  1    1
2  2    1
dtype: int64
```
#### Expected Output

```
Out[4]:
0    1
1    1
dtype: int64
```

(this is what you get with a unique `by` column -- 

```
a.groupby(['a'], as_index=False).apply(lambda x: 1)
Out[8]: 
0    1
1    1
dtype: int64
```
#### output of `pd.show_versions()`

pd.show_versions()
## INSTALLED VERSIONS

commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 15.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.1
setuptools: 20.3
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.1.2
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.2
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.1
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
pandas_datareader: None
