Categorical type lost during pd.merge ?


```python
df1=pd.DataFrame({'categorical':['A','A','B'],'i':[1,2,3]})
df1['categorical']=df1['categorical'].astype('category')
df2=pd.DataFrame({'notcategorical':['A','A','B'],'i':[1,2,3]})
df1.dtypes,df2.dtypes
#shows categorical type

#But:
df3=pd.merge(df1,df2,on='i')
df3.dtypes
#categorical now an object type?
```
#### Problem description

Merging a dataframe containing a categorical column results in that column losing its `category` type in the merged dataframe.

#### Expected Output

The `category` type of column is preserved in the merged output dataframe.

#### Version

`pandas: 0.19.2`
