Use XML export of simple forms of riak_cs_xml:to_xml in multipart uploads
Use XML export of simple forms of riak_cs_xml:to_xml instead of export_xml/1
Fix bug of accept content type
Add test case of listing users which has non-ASCII UTF-8 characters

This is the case of #628.
Fix has already been made by the commit d215335577a3ef02510bc8d115fe187f79eced8c