Revert "Fixed a typo ("return _nan" -> "return_nan")"

This reverts commit 01c833a.

Author: florian

category_encoders/ordinal.py

@@ -144,7 +144,7 @@ def fit(self, X, y=None, **kwargs):
         self.mapping = categories
 
         X_temp = self.transform(X, override_return_df=True)
-        self.feature_names = X_temp.columns.values.tolist()
+        self.feature_names = X_temp.columns.tolist()
 
         # drop all output columns with 0 variance.
         if self.drop_invariant: