MULE-8018: Merge http connector code cleanup

It isn't about it's name matching, needing it or the infrastructure around it, it's about creating MuleMessages in a way consistent with the rest of Mule.  By doing this things are simpler to understand, more consistent, we have less repeated code and if we do need to build infrastructure around it for example to implement a cross-cutting concerns then we are safe in the knowledge thats it's being used consistently.  I can't force you to understand the value of architectural consistency via github comments, so I won't continue to comment on this here.