Fix order dependent AP test

any_instance.stubs + unstub with Mocha doesn't restore the original status in the following case,
so we need to undef Customer#to_json before every test

require 'test/unit'
require 'mocha/setup'

module M
  def foo() :foo; end
end

class C
  include M

  undef_method :foo
end

C.any_instance.stubs(:foo).returns(:mocha)

C.any_instance.unstub(:foo)

actionpack/test/controller/mime_responds_test.rb

@@ -646,6 +646,7 @@ class RespondWithControllerTest < ActionController::TestCase
     Mime::Type.register_alias('text/html', :iphone)
     Mime::Type.register_alias('text/html', :touch)
     Mime::Type.register('text/x-mobile', :mobile)
+    Customer.send(:undef_method, :to_json) if Customer.method_defined?(:to_json)
   end
 
   def teardown