remain backwards compatible, exception can be constructed with no arguments

actionpack/lib/action_controller/metal/exceptions.rb

@@ -5,7 +5,9 @@ class ActionControllerError < StandardError #:nodoc:
   class BadRequest < ActionControllerError #:nodoc:
     attr_reader :original_exception
 
-    def initialize(type, e)
+    def initialize(type = nil, e = nil)
+      return super() unless type && e
+
       super("Invalid #{type} parameters: #{e.message}")
       @original_exception = e
       set_backtrace e.backtrace