BUG: #19497 FIX. Add tupleize_cols option to internals._transform_index()
- Closes #19497
- NO TESTS have been added/passed. Currently experiencing issues getting pandas conda environment installed so unable to do this myself. Very simple alteration however...
- Code passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- Implements tupleize_cols option for pandas.core.internals_transform_index(), and provides the capability to manipulate this option using pandas.DataFrame.rename(). 
**NOTE**: for renaming, the keyword argument default is False, because as suggested [here](https://github.com/pandas-dev/pandas/issues/19497), this is unintuitive and unexepected. The keyword argument for the _transform_index() is True however, which is consistent with default pandas.Index() creation behaviour.
