fix handling of If-Modified-Since if If-None-Match is present (don't return 412 for date parsing errors); follow current draft for HTTP/1.1, which tells us to ignore If-Modified-Since if we have matching etags.

See:
 http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-21#section-5
 > it makes sense to ignore the If-Modified-Since when entity tags are
 > understood and available for the selected representation.

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2865 152afb58-edef-0310-8abb-c4023f1b3aa9