Add support for Delegation Handler

This is a pretty simple handler to support delegation. I had to modify the CheckerServletRequest to support mutating headers.
No, I suppose not. Java documentation states that the value of the "first" header in the response should be returned. Maybe I'll make the order start with the wrapped request, then look into the map in the wrapper. That also highlights another flaw in these changes. The map being maintained should be a use a value List to preserve ordering and allow duplicates. I'll replace that tomorrow as well.