DataFrame.interpolate(axis='columns') throws exception while DataFrame.interpolate(axis=1) not
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd

x = np.linspace(0,100,1000)
y = np.sin(x)
df = pd.DataFrame(data=np.tile(y, (10,1)), index=np.arange(10), columns=x)
df.reindex(columns=x*1.005).interpolate(method='linear', axis=1)

df.reindex(columns=x*1.005).interpolate(method='linear', axis='columns')
```
#### Problem description

The interpolation with argument `axis` set to 'columns' throws an exception from 
https://github.com/pandas-dev/pandas/blob/c460a929e19535335085c652e7e35eee1d9c7c2e/pandas/core/generic.py#L6800

The exception is:
```
UnboundLocalError: local variable 'ax' referenced before assignment
```

The same method for parameter `axis=1' works fine.

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-44-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.1
pytest: None
pip: 18.1
setuptools: 40.6.3
Cython: None
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: 0.11.2
IPython: 7.2.0
sphinx: None
patsy: None
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.2.5
bs4: None
html5lib: None
sqlalchemy: 1.2.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
