Mime types: use 'application/xml' instead of 'text/xml'
Mime types: use 'application/xml' instead of 'text/xml'

According to RFC 7303, the latter is now an alias of the former type.
However, that same document recommends the use of `application/xml` "to
avoid possible confusion based on the earlier distinction".

kazu-yamamoto/wai,kazu-yamamoto/wai,yesodweb/wai,yesodweb/wai