Rolling with offset and axis=1
#### Code Sample

Here I have a DataFrame with a DatetimeIndex for the columns:
```python
cols = pd.date_range('2019-8-01', '2019-08-07', freq='D')
data = np.ones((5, len(cols)))
df = pd.DataFrame(data, columns=cols)
df.rolling('1d', axis=1).sum()
```
The above gives 'ValueError: window must be an integer'

Here I have a DataFrame with a DatetimeIndex for the rows:
```python
df2 = df.transpose()
df2.rolling('2d', axis=0).sum()
```
However this works totally fine.

#### Problem description
I believe there may be a bug: If I have a DatetimeIndex in the columns, specifying an offset for df.rolling does not work but gives a value error (that the window must be an integer). It works however as expected if I transpose the dataframe.

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.1
numpy            : 1.16.4
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.2
setuptools       : 41.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.7.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.0
numexpr          : None
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None

</details>
