MessageMarshaller abstracts content-type header.

Change-Id: I5f400ef33553f80280082c7b911741d3a5079700
Reviewed-on: http://review.couchbase.org/36136
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>