NDFrame.interpolate(): variable 'ax' not assigned when axis='index'
The `NDFrame.interpolate` function fails when passing a string as axis. Example:

```python
>>> import numpy as np
>>> import pandas as pd
>>> df = pd.DataFrame(np.zeros((3,2)), columns=['a','b'])
>>> df.iloc[1] = np.nan
>>> df
     a    b
0  0.0  0.0
1  NaN  NaN
2  0.0  0.0
>>> df.interpolate(axis=0)
     a    b
0  0.0  0.0
1  0.0  0.0
2  0.0  0.0
>>> df.interpolate(axis='index')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/lehanson/anaconda3/envs/plots/lib/python3.7/site-packages/pandas/core/generic.py", line 7006, in interpolate
    ax = _maybe_transposed_self._get_axis_number(ax)
UnboundLocalError: local variable 'ax' referenced before assignment
```


From the documentation and from the function itself, it looks like `df.interpolate(axis='index')` was intended to work, but that maybe someone accidentally deleted a line in generic.py? The function seems to work properly if I add `ax = axis` in the else block here: 
https://github.com/pandas-dev/pandas/blob/171c71611886aab8549a8620c5b0071a129ad685/pandas/core/generic.py#L6998-L7006


I am using pandas version 0.25.1