fix_hash_subclass.rb

module InheritsFromHash
  def select(*args, &block)
    dup.tap { |hash| hash.select!(*args, &block) }
  end

  def reject(*args, &block)
    dup.tap { |hash| hash.reject!(*args, &block) }
  end
end

class HashSubclass < Hash
  include InheritsFromHash
  
  def foo
    self.reject {|k,v| k[:foo]}
  end
end

obj = HashSubclass.new
new_obj = obj.foo
new_obj.class

# In ruby 1.9.3
=> HashSubclass

# In Ruby 2.2+
=> HashSubclass