when the server provides an Expires header, our cache extension may not work everywhere

[GoogleCodeExporter]

In some configurations, Apache will add an Expires header to resources.

This can conflict with our cache extension headers, which use Cache-Control: 
max-age.

In most cases this is a non-issue since CC max-age is preferred to Expires per 
the HTTP/1.1 RFC.

However Firefox (through v4?) and HTTP/1.0 proxies will use Expires when both 
are present.

When an Expires header is present, we should overwrite it with a date that 
matches the value in the CC max-age header.

Original issue reported on code.google.com by bmcqu...@google.com on 6 Nov 2010 at 11:57