Cant process content types with charset

[ErwinMO4]

Any request with a header of the form "application/json; charset=utf-8" will result in a 500 return code, because _get_body fails. The reason for this is that header_parts[0] (wsgi_adapter line 60) equals application/json (in my example), which does not contain any equality sign.

As this library is depricated I will not be spending any extra time on a pull request, but running the split function for only parts with at least one = should resolve the bug.