Fix for UnicodeEncodeError when form data includes an Unicode character

After getting lots of "'latin-1' codec can't encode characters in position 62-66: ordinal not in range(256)" when POSTing valid Unicode characters with "multipart/form-data" requests I figured out that the issue is that '_perform_form_overloading()' is trying to encode the form data as with 'iso-8859-1' which fails.