Keep relative URLs relative after rewrite.

[GoogleCodeExporter]

What steps will reproduce the problem?
http://foo.com/ is the front end which makes sub-requests to http://bar.com/ 
(the backend which should only be accessed from foo.com).

What happens?

mod_instaweb is installed on bar.com and so mod_pagespeed absolutifies URLs to 
http://bar.com/... which is not accessible from the user.

What we want to happen?

All sub-requests should go to foo.com . One simple solution would be to leave 
all resources which were originally relative URLs, still relative. A more 
complicated solution would be to implement domain mappings, telling us to move 
all resources from bar.com to foo.com .

Original issue reported on code.google.com by sligocki@google.com on 17 Nov 2010 at 8:12