Respect Vary headers for resources

```
Currently mod_pagespeed will rewrite & cache resources with Vary:User-Agent and 
Vary:Referer headers.  It probably shouldn't.

An alternative strategy would be to put the user-agent into the cache-key, but 
I'm afraid this will add a lot of cache pressure, in terms of file-system use 
and capacity.
```

Original issue reported on code.google.com by `jmara...@google.com` on 14 Mar 2011 at 9:33
