Validate fake iOS 5.1.1 user agents by checking for webp in the Accept header.

No Safari version supports webp at the time of writing.
Whitelist feedly requests for missing Accept header.

User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_2) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.77 Safari/535.7 AppEngine-Google; (+http://code.google.com/appengine; appid: s~feedly-social)