Set attribute transformed_names_ when unpickling

Resolves #127.

Author: dukebody

sklearn_pandas/dataframe_mapper.py

@@ -150,6 +150,7 @@ def __setstate__(self, state):
         self.input_df = state.get('input_df', False)
         self.built_features = state.get('built_features', self.features)
         self.built_default = state.get('built_default', self.default)
+        self.transformed_names_ = state.get('transformed_names_', [])
 
     def _get_col_subset(self, X, cols, input_df=False):
         """