MULE-8018: Merge http connector code cleanup

I don't think that using MuleMessageFactory is wrong because it is "tied" to transports.
I more concerned about not having a common way to create messages for a transport/connector. This class is not just creating messages, but events. And the method signature contains many things that are tied to HTTP, so it would be hard to make it general in the future if we continue creating custom classes.
If MuleMessageFactory has four methods, can we extract the method we would need into a different interface and make MuleMessageFactory extend from it?