[Reviewer Rob]Fixes issue 349.  Set correct reason on 403 response when AV lookup fails

Rob

Can you review my fix to the issue you flagged with Sprout setting the wrong reason on some 403 responses.  The fix is fairly self-explanatory.  I tested by updating the UT RespMatcher to check the reason text, verified that this reproduced the problem, then ran the tests again with the fix.  I also fixed a build warning in the custom_headers_test UTs as it was bugging me.

Mike