Fix return values for authenticate in docs and add present? to check nil values

Author: kaushikhande

actionpack/lib/action_controller/metal/http_authentication.rb

@@ -211,7 +211,7 @@ def request_http_digest_authentication(realm = "Application", message = nil)
         end
       end
 
-      # Returns false on a valid response, true otherwise.
+      # Returns true on a valid response, false otherwise.
       def authenticate(request, realm, &password_procedure)
         request.authorization && validate_digest_response(request, realm, &password_procedure)
       end