The ProxyHandler does not update the HOST header with the new host

When using the ProxyHandler the HOST header should be updated to utilize the host where the proxied request will go. The original HOST header value should be retained in the X-Forwarded-Host header.

This is per the Http 1.1 spec (http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html) section 14.23

Example:

Incoming request:
GET /ping HTTP/1.1
Host: somewhere.somecompany

Proxied Request:
GET /ping HTTP/1.1
Host: somewhere.internal.somecompany
X-Forwarded-Host: somewhere.somecompany