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