Categorical Series.describe() returns wrong dtype
`Series.describe()` seems to return the wrong dtype for series with categorical data. See example below:

```python
import pandas as pd

series = pd.Series(pd.Categorical(values=[1], categories=[1, 2]))
print(series.describe())
# prints: ...  dtype: int64
print(series.dtype)
# prints: category

```

This is at least confusing. Example has been produced using pandas 0.19.1 and Python 3.5.2.
