handle None and empty strings properly
Created originally on Bitbucket by cmcdaniel (Craig McDaniel)
Was already merged in Bitbucket before import, marked as merged by the import user
This change is compatible with the wsme changes just made. None is recognized as None using the xsi:nil attribute and empty strings are handled properly as well. All unit tests pass including the new test_settext_empty and test_settext_none.