Inline images sent to browsers that don't support inline images

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Use a browser that doesn't support inline images - most commonly browsers on 
mobile devices (Issue noted on a PSP & Nokia mobile phone) other than IE6/7.
2. Go to a site that has rewrite_images enabled, and has small images that are 
suitable for inlining. (Issue occurs on a phpBB forum - emoticons, topic icons 
and reply/quote/PM buttons aren't shown)

What is the expected output? What do you see instead?
Expected: The various icons are displayed
Occurred: The icons aren't shown. Instead a "missing image" icon is displayed 
instead.

What version of the product are you using (please check X-Mod-Pagespeed header)?
0.9.15.3-404

On what operating system?
CentOS 5.3 (OpenVZ VPS)

Which version of Apache?
2.2.3

Which MPM?
Prefork

Please provide any additional information below, especially a URL or an HTML 
file that exhibits the problem.
http://twokinds.net/forum/

Issue likely occurs as a result of 
http://www.google.com/codesearch/p?hl=en#kGe7pYmmq7E/trunk/src/net/instaweb/rewr
iter/img_rewrite_filter.cc&d=4&l=327 - apparently, the only user agent check 
done is a check if the browser is IE6/7 or not. 
Relevant thread here: 
http://groups.google.com/group/mod-pagespeed-discuss/browse_thread/thread/160bed
6b047009c6

Original issue reported on code.google.com by kyl...@twokinds.net on 8 Mar 2011 at 6:49