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
```

sure, pls add a note in 0.17.1 whatsnew
 @sinhrks looks good, can you rebase / add a release note 
 Rebased, and added release note. Followings are asv results added in this PR.

```
    before     after       ratio
  [a89b96d4] [b2cb56a8]
    10.76ms    10.60ms      0.99  series_methods.series_dropna_datetime.time_series_dropna_datetime
-   12.07ms   726.61μs      0.06  series_methods.series_dropna_int64.time_series_dropna_int64
```
 thanks!
