Series.str.cat() with NaN in series returns NaN, rather than ignoring NaN
I find this surprising as the rest of the pandas `Series.str.*` API ignores NaN values.

``` python
In [1]: import pandas as pd

In [2]: import numpy as np

In [3]: pd.__version__
Out[3]: u'0.17.0'

In [4]: s = pd.Series(['asdf','sdfg',np.nan,'qwer','wert'])

In [5]: s.str.cat(sep=' ')
Out[5]: nan
```

I think this should return

``` python
In [5]: s.str.cat(sep=' ')
Out[5]:'asdf sdfg qwer wert'
```
