BUG: astype('category', categories=...) failes on a series of categorical type
`s.astype('category', categories=['a', 'b', 'c'])` fails when the series is already of Categorical dtype:

> TypeError: _astype() got an unexpected keyword argument 'categories'

 I am not sure if this _should_ work (it would then be equivalent to `set_categories`?), but in any case the current error message is not informative:

```
In [49]: s = pd.Series(['a', 'b', 'a'])

In [50]: s
Out[50]:
0    a
1    b
2    a
dtype: object

In [51]: s.astype('category')
Out[51]:
0    a
1    b
2    a
dtype: category
Categories (2, object): [a, b]

In [52]: s.astype('category', categories=['a', 'b', 'c'])
Out[52]:
0    a
1    b
2    a
dtype: category
Categories (3, object): [a, b, c]

In [53]: scat = s.astype('category')

In [54]: scat.astype('category', categories=['a', 'b', 'c'])
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-54-f955e6286a85> in <module>()
----> 1 scat.astype('category', categories=['a', 'b', 'c'])

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\generic.pyc in astype(self, dty
pe, copy, raise_on_error, **kwargs)
   2415
   2416         mgr = self._data.astype(
-> 2417             dtype=dtype, copy=copy, raise_on_error=raise_on_error, **kwa
rgs)
   2418         return self._constructor(mgr).__finalize__(self)
   2419

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\internals.pyc in astype(self, d
type, **kwargs)
   2516
   2517     def astype(self, dtype, **kwargs):
-> 2518         return self.apply('astype', dtype=dtype, **kwargs)
   2519
   2520     def convert(self, **kwargs):

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\internals.pyc in apply(self, f,
 axes, filter, do_integrity_check, **kwargs)
   2471                                                  copy=align_copy)
   2472
-> 2473             applied = getattr(b, f)(**kwargs)
   2474
   2475             if isinstance(applied, list):

c:\users\vdbosscj\scipy\pandas-joris\pandas\core\internals.pyc in astype(self, d
type, copy, raise_on_error, values, **kwargs)
    371     def astype(self, dtype, copy=False, raise_on_error=True, values=None
, **kwargs):
    372         return self._astype(dtype, copy=copy, raise_on_error=raise_on_er
ror,
--> 373                             values=values, **kwargs)
    374
    375     def _astype(self, dtype, copy=False, raise_on_error=True, values=Non
e,

TypeError: _astype() got an unexpected keyword argument 'categories'

```
