title: numeric observable not working with knockout validation
body: <p>I'm using a custom observable, <code>numericObservable</code>, based on <a href="https://stackoverflow.com/a/7396039/791406">this</a>, which forces knockout to serialize numeric fields as numbers in json, not strings; I've got that part working.</p>
  <p>I've added validation to enforce <code>number</code> as the field type for port:</p>
  <pre><code> self.port = ko.numericObservable(22).extend({ number: true });
  </code></pre>
  <p>..but validation is not working 100%. It seems to pick up non-numerics sometimes, other times it just fails to recognize. If I clear the value for port, switch focus to another field, then come back and enter a number, it works.</p>
  <h3>Fiddle</h3>
  <p><a href="http://jsfiddle.net/SAFX/q4QCY/13/" rel="nofollow noreferrer">http://jsfiddle.net/SAFX/q4QCY/13/</a></p>
  <p>If I use validation against a plain observable, like below, it works, so I suspect the issue may be with extending <code>numericObservable</code>. Also, the <a href="http://jsfiddle.net/rniemeyer/RJbdS/" rel="nofollow noreferrer">example</a> from which I got <code>numericObservable</code> uses <code>ko 2.2.2</code>, my fiddle uses <code>ko 3.0.0</code>.</p>
  <pre><code>//works
  self.port = ko.observable(22).extend({ number: true });
  </code></pre>