str.split ignores n if pat is None

as observed in: http://stackoverflow.com/questions/21998590/
`str.split` ignores `n` if `pat` is `None`

relevant source code:

```
# pandas/core/strings.py
def str_split(arr, pat=None, n=None):
    if pat is None:
        if n is None or n == 0:
            n = -1
        f = lambda x: x.split()
    ....
```
