BUG: interpolate with no nans and limit
Closes https://github.com/pydata/pandas/issues/7173

I was assuming that there would be at least one NaN when I was adjusting for the limits. Pretty simple fix. 
Here's the intended behavior:

``` python
In [1]: s = pd.Series([1., 2, 3])

In [2]: s.interpolate()
Out[2]: 
0    1
1    2
2    3
dtype: float64

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

thanks @TomAugspurger 
