SLING-3141 : AbstractAuthenticationFormServlet should make sure resource is a valid redirect

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1528062 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' of http://git.apache.org/sling into sling-3140
SLING-3140 - made the cast based conversion the fallback