PERF: Series.dropna with non-nan dtype blocks
Minor improvement when `dropna` actually does nothing. If OK for 0.17, I'll add a release note.

```
import numpy as np
import pandas as pd

s = pd.Series(np.random.randint(0, 1000, 50000000))

# before
%timeit s.dropna()
#1 loops, best of 3: 1.58 s per loop

# after
%timeit s.dropna()
#1 loops, best of 3: 568 ms per loop
```
