:on is removed from options before the Mapping is instantiated, so remove it from IGNORE_OPTIONS

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -60,7 +60,7 @@ def constraint_args(constraint, request)
       end
 
       class Mapping #:nodoc:
-        IGNORE_OPTIONS = [:on, :constraints, :defaults, :only, :except, :shallow, :shallow_path, :shallow_prefix]
+        IGNORE_OPTIONS = [:constraints, :defaults, :only, :except, :shallow, :shallow_path, :shallow_prefix]
         ANCHOR_CHARACTERS_REGEX = %r{\A(\\A|\^)|(\\Z|\\z|\$)\Z}
 
         attr_reader :scope, :options, :requirements, :conditions, :defaults