adds model for X-Forwarded-Host header

[pektinasen]

Solves one part of #5010

[rossabaker]

I can't tell whether this should include a port or not. MDN defines it ambiguously. It also says "The standardized version of this header is Forwarded", whose for value does support a port. And if it's supposed to represent the original Host header, that comes with an optional port. I can find other references to an X-Forwarded-Port header, which speaks to the lack of standardization.

My gut feeling is that this should have a port and look just like the Host header model, but I lack real-world experience on this header, and would accept it as is if you disagree.