title: Add test cases to test set using Ruby Rally Rest API
body: <p>I'm trying to add test cases to a test set in Rally.</p>
  
  <p>I have the test set object and the test case object.
  I can REPLACE the entire test set list with THIS test case object and it works fine.
  But I really need to ADD this test case to the list.  </p>
  
  <p>Any idea how I do this differently?</p>
  
  <p>testset_object.update(:test_cases => test_case_object)</p>