Serializer::Mutable: Fix reponse content-type

curl -X PUT -k -i \
 -H 'Accept: application/json; charset=UTF-8' \
 -H 'Content-Type: text/xml; charset=UTF-8' \
 -d '<r><elt1>content1</elt1></r>'

failed to find the correct RESPONSE serializer.
Content-Type is the format of the input while Accept is the format for
the response data (output).

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>