Groupby with Grouper + ffill results in Column being replaced with BinGrouper Objects
#### Code Sample, a copy-pastable example if possible

```python

df = pd.DataFrame({'dt': ['2017-01-01', '2017-01-02', '2017-01-03'], 'id': [1, 1, 2], 'data': [1.0, np.nan, 3.1]})

df['datetime'] = pd.to_datetime(df.dt)

df.groupby(['id', pd.Grouper(key='datetime', freq='2D')]).ffill()
```
output:
```
   id                                           datetime          dt  data
0   1  <pandas.core.groupby.groupby.BinGrouper object...  2017-01-01   1.0
1   1  <pandas.core.groupby.groupby.BinGrouper object...  2017-01-02   1.0
2   2  <pandas.core.groupby.groupby.BinGrouper object...  2017-01-03   3.1
```
#### Problem description

the datetime column is being replace with a `BinGrouper` object when I `ffill` a `Grouper`ed groupby DataFrame


#### Expected Output

          dt        id  data   datetime
    0  2017-01-01   1   1.0     2017-01-01
    1  2017-01-02   1   1.0     2017-01-02
    2  2017-01-03   2   3.1     2017-01-03

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.3
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.2
numpy: 1.14.1
scipy: None
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.10
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
