BUG: Fixes #2917: numpy.lib._iotools.StringConverter.upgrade return

I'm new around, so let me know if I should be doing something differently.

I've checked for coverage and PEP8-compliance.
@rgommers, thanks for reviewing my code!  So, should I get rid of those blank lines?  I put the extra lines because I thought those were top-level functions and PEP 8 says "Separate top-level function and class definitions with two blank lines".  Also, the pep8 command line tool was complaining about the single blank line.