assert_frame_equal fails when dtype is categorical
```
assert_frame_equal(desired_df, df)
  File "C:\Anaconda\lib\site-packages\pandas\util\testing.py", line 1049, in assert_frame_equal
    obj='DataFrame.iloc[:, {0}]'.format(i))
  File "C:\Anaconda\lib\site-packages\pandas\util\testing.py", line 927, in assert_series_equal
    assert_attr_equal('dtype', left, right)
  File "C:\Anaconda\lib\site-packages\pandas\util\testing.py", line 721, in assert_attr_equal
    result = left_attr == right_attr
TypeError: data type not understood
```
