Only use valid mime type symbols as cache keys

CVE-2013-6414

actionview/lib/action_view/lookup_context.rb

@@ -62,6 +62,13 @@ module ActionView
       @details_keys = ThreadSafe::Cache.new
 
       def self.get(details)
+        if details[:formats]
+          details = details.dup
+          syms    = Set.new Mime::SET.symbols
+          details[:formats] = details[:formats].select { |v|
+            syms.include? v
+          }
+        end
         @details_keys[details] ||= new
       end