BUG/API: interpolate with limit=0 should mean no interpolation
Current behavior:

```
In [28]: s = pd.Series([1, 2, np.nan, np.nan, 5])

In [29]: s
Out[29]:
0     1
1     2
2   NaN
3   NaN
4     5
dtype: float64

In [30]: s.interpolate(limit=0)
Out[30]:
0    1
1    2
2    3
3    4
4    5
dtype: float64
```

But in fact, `limit=0` in contrast to `limit=None` should mean no interpolation.
