Do not serve source map on Hash mismatch

[GoogleCodeExporter]

Source maps are served by the same process as rewritten resources. By default, 
if we get a request for a rewritten resource with a hash different from the one 
we have in cache, we'll still serve the one in cache (with 
Cache-Control:private,max-age=300).

This is very bad for source maps, because ones with different hashes are 
completely wrong. We should not serve these.

Original issue reported on code.google.com by sligocki@google.com on 21 Apr 2014 at 7:21

[sligocki]

Fixed by fb49739