BUG: groupby.agg coercing booleans
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

dat = pd.DataFrame({'a': [1, 1, 2, 2], 'b': [0, 1, 2, 3], 'c': [None, None, 1, 1]})

gp = dat.groupby('a')

case_one = gp['b'].aggregate(lambda x: (x != 0).all())

case_two = gp['c'].aggregate(lambda x: x.isnull().all())

```
#### Problem description
By comparing ``case_one`` and ``case_two``, it seems that ``aggregate`` function turns boolean values sometimes to integers 1 or 0.

For ``case_one`` we get,
```
a
1    False
2     True
Name: b, dtype: bool
```
However for ``case_two`` we get
```
a
1    1.0
2    0.0
Name: c, dtype: float64
```
#### Expected Output
For ``case_two`` we should get

```
a
1    True
2     False
Name: b, dtype: bool
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-53-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8
LOCALE: en_AU.UTF-8

pandas: 0.19.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: None
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
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: 2.8
boto: None
pandas_datareader: None

</details>

This same issue appears in a different environment as well.

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-53-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 9.0.1
setuptools: 23.0.0
Cython: None
numpy: 1.11.2
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.0.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.6.1
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: 1.1.3
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
