pd.Series.interpolate(method='spline') Error
Although in docs it says `method='spline'` is a choice, but it raises a `ValueError`.

```
import numpy as np
import pandas as pd

np.random.seed(1)
s = pd.Series(np.arange(10)**2)
s[np.random.randint(0,9,3)] = np.nan
s.interpolate(method='spline')

# raise ValueError
ValueError: invalid method 'spline' to interpolate.
```
