DataFrameMapper with df_out should preserve categorical data

[mratsim]

DataFrameMapper transformation removes the "category" dtype from dataframe columns

Categorical status can be checked with:
hasattr(df['categorical_column'], 'cat')

Some classifiers like LightGBM can auto detect categorical data from dataframes and handle them very efficiently without OneHotEncoding.

[MarcusJones]

Just ran into this one, I wrote a custom transformer specifically to convert columns to 'category' dtype. Had no idea why it wasn't working until I found this issue. Would this be difficult to implement?