rhbz831479 - REST return 400 bad request for invalid extensions

https://bugzilla.redhat.com/show_bug.cgi?id=831479

Integration test is written but if you want to manually test it, boot up your local instance and try(replace project/iteration/docId/locale to suit):

curl "http://localhost:8080/zanata/rest/projects/p/about-fedora/iterations/i/master/r/About_Fedora/translations/zh?ext=blah" -H "Cache-Control: no-cache" -H "X-Auth-Token: b6d7044e9ee3b2447c28fb7c50d86d98" -H "X-Auth-User: admin" -H "User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.31 (KHTML, like Gecko) Chrome/26.0.1410.63 Safari/537.31"