difference between two hashes

hash_minus.rb

class Hash
  def -(h)
    reject do |k, v|
       h.has_key?(k) && (v == h[k])
    end
  end
end

describe Hash, 'difference between two hashes' do
  it "should return a hash" do
    ({:a => 1, :b => 2} - {:a => 1}).should == {:b => 2}
    ({:a => 2, :b => 2} - {:a => 1}).should == {:b => 2, :a => 2}
    ({:a => 1, :b => 2} - {:c => 1}).should == {:a => 1, :b => 2}
    ({:a => ''} - {:a => 'not empty'}).should == {:a => ''}
  end
end