mod_xml2enc: cut out inappropriate fallback (and misleading log message)

when we've failed to detect a charset but successfully fallen back to
config default.
Analysis by Tim Skopnik, patch by niq.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1785780 13f79535-47bb-0310-9956-ffa450edef68