BUG: Series with Categorical and dtype
xref https://github.com/pydata/pandas/pull/12573#discussion_r55561598

``` python
In [37]: pd.Series(pd.Categorical([1,2,3]), dtype='category')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-37-87a1a3228158> in <module>()
----> 1 pd.Series(pd.Categorical([1,2,3]), dtype='category')

ValueError: cannot specify a dtype with a Categorical
```
