non-caching-related headers are stripped from resources

[GoogleCodeExporter]

When mod_pagespeed rewrites a resource, it generates a complete fresh set of 
response-headers so the caching is correct.  But it should retain any 
non-caching-related response-headers in the response.

E.g. if a site has this in a conf file:

           Header append 'DEV' '1'

The origin resource will include "DEV:1" but not the rewritten resource.

Original issue reported on code.google.com by jmara...@google.com on 20 Jul 2011 at 3:20