adds Object#subclasses

subclass.rb

class Object

  # A method that adds a #subclass class method allowing
  # you to find the subclasses of a particular class.
  #
  # @example
  #   
  #   class String2 < String; end
  #   class String3 < String; end
  #   String.subclasses
  #   #=> [:String2, :String3]
  #
  #   class String2Sub < String2; end
  #   String2.subclasses
  #   #=> [:String2Sub]
  #   String.subclasses
  #   #=> [:String2, :String3, :String2Sub]
  #
  # @return [Array[Symbol]]
  #
  def self.subclasses
    Module.constants.find_all do |c_klass| 
      if c_klass != c_klass.upcase
        self > (Object.const_get(c_klass))
      else
        nil
      end
    end
  end
end