Provide easier mechanism to flush server cache.

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use an image with 1 hour TTL
2. Load HTML page with the image until it is rewritten
3. Change the image
4. Reload HTML page and notice that image hasn't changed (probably cached 
locally)
5. Shift-Reload page and notice that image still hasn't changed (probably still 
in mod_pagespeed cache)

What is the expected output? What do you see instead?

Without mod_pagespeed, Shift-Reload will flush the cache. It would be nice if 
mod_pagespeed also did that.

Please use labels and text to provide additional information.

From simple experimentation on Chrome, it appears that using Shift-Reload:
* changes "Cache-Control: max-age=0" -> "Cache-Control: no-cache" and 
* adds "Pragma: no-cache"

If we see these perhaps we should flush our cache (at least for these files).

Original issue reported on code.google.com by sligocki@google.com on 4 Dec 2010 at 6:53