text: Title: Test that transform_column does not mutate original
  Body: Linked from issue #645.
  
  > Should be put a test somewhere for some garantee?
  > 
  > _Originally posted by @VPerrollaz in https://github.com/ericmjl/pyjanitor/issues/645#issuecomment-630170156_
  
  Possible test outline (just pseudo code, don't treat it as real code):
  
  ```python
  def test_transform_column_does_not_mutate_original():
      df_transformed = df.transform_column("some_col", some_func)
      with pytest.raises(AssertionError):
          pd.testing.assert_frame_equal(df, df_transformed)
  ```