title: How to partially mock an object in javascript / node?
body: <p>I'm using Sinon, and want to mock only some of the methods of an object, but the mock returns undefined for all methods.  Is there anyway to get the calls to pass through by default?</p>
  
  <pre><code>var sinon=require('sinon');
  var util = require('util');
  var mockedUtil = sinon.mock(util);
  
  util.isNumber(1); // true
  mockedUtil.isNumber(1); // exception because isNumber is undefined
  </code></pre>