Regression for pd.Series.replace(np.nan, inplace=True)
This code was working in pd 0.12.0, but is failing in current master ('0.12.0-955-ga96cd66'):

``` python
df.last_el_cmd.replace(np.nan, inplace=True)
```

The error:

``` python
/usr/local/epd/lib/python2.7/site-packages/pandas-0.12.0_955_ga96cd66-py2.7-linux-x86_64.egg/pandas/core/generic.pyc in replace(self, to_replace, value, inplace, limit, regex, method, axis)
   1903             if not is_dictlike(to_replace):
   1904                 if not is_dictlike(regex):
-> 1905                     raise TypeError('If "to_replace" and "value" are both None'
   1906                                     ' and "to_replace" is not a list, then '
   1907                                     'regex must be a mapping')

TypeError: If "to_replace" and "value" are both None and "to_replace" is not a list, then regex must be a mapping
```

I saw that some filter related stuff was updated to use regex for 0.13dev, but I don't understand why this makes my command failing? 
