title: A function similar to $.extend but adds no new attributes
body: <p>I think I have seen such a jquery function, but cannot find it.</p>
  
  <pre><code>var x = {a:1, b:2};
  var y = {a:5,     c:3};
  
  var z = $.extend({}, x, y);   // {a: 5, b: 2, c: 3}
  
  // want a similar jquery function which returns {a: 5, b: 2}
  // that is, returns only attributes which x has (a,b), but adds no new attributes (c)
  </code></pre>