loffset in pd.Grouper does not apply 
#### Code Sample

```python

date_index = pd.date_range("2019-08-26", "2019-09-20")
sessions = DataFrame(range(1, len(dates) + 1), index=date_index, columns=['foo'])

sessions.groupby(pd.Grouper(freq=pd.Timedelta(weeks=4), key='created_at', loffset='4W')).size()

```
#### Problem description

The above use of `loffset` does not change the grouping labels as expected. The corresponding code using `DataFrame.resample` does work. Try:

```python
sessions.resample(pd.Timedelta(weeks=4), on='created_at', loffset='4W')
```

#### Expected Output

```
created_at
2019-09-22    1048
2019-10-20     904
Freq: 28D, dtype: int64
```

#### Actual Output 

```
created_at
2019-08-26    1048
2019-09-23     904
Freq: 28D, dtype: int64
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.6.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.25.2
numpy            : 1.16.2
pytz             : 2018.9
dateutil         : 2.8.0
pip              : 9.0.1
setuptools       : 28.8.0
Cython           : None
pytest           : 4.6.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : 1.1.1
lxml.etree       : 4.3.2
html5lib         : None
pymysql          : 0.9.3
psycopg2         : None
jinja2           : 2.10
IPython          : 7.5.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.3.2
matplotlib       : 3.0.0
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.2.1
sqlalchemy       : 1.2.12
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : 1.1.1

</details>
