JSON.dump({}) raise NoMethodError

This code raise NoMethodError.

  require 'rubygems'
  require 'activesupport'
  JSON.dump({}) # raise NoMethodError

but,

  require 'rubygems'
  require 'json'
  JSON.dump({}) # => "{}"

In 'active_support/json/encoders/hash.rb' Hash#as_json or Hash#to_json is wrong.
I attached a simple patch for this problem.