Revert "Reverted OrdinalEncoder possible bug:"

This reverts commit b95f569.

Author: florian

category_encoders/ordinal.py

@@ -323,7 +323,7 @@ def ordinal_encoding(X_in, mapping=None, cols=None, handle_unknown='value', hand
                 if handle_missing == 'value' and ~data.index.isnull().any():
                     data.loc[nan_identity] = -2
                 elif handle_missing == 'return_nan':
-                    data.loc[nan_identity] = -2
+                    data.loc[nan_identity] = np.nan
 
                 mapping_out.append({'col': col, 'mapping': data, 'data_type': X[col].dtype}, )