title: knockout the "check" binding not working
body: <p>I am new to knockout, I have found an example of checkbox binding which could be very useful in my website.
  Link to the example: <a href="http://knockoutjs.com/documentation/checked-binding.html" rel="nofollow">http://knockoutjs.com/documentation/checked-binding.html</a></p>
  
  <p>I tried to apply it to my page but it didn't work, then I tried to copy this example without changing anything but still it didn't work. I tried it on different browser(Chrome, Firefox, IE) and including different version of knockout libraries, but still it didn't work.
  I have no other idea what I am doing wrong, please help!</p>
  
  <p>Here is the code of this example:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;head&gt;
  &lt;script type='text/javascript' src='http://ajax.aspnetcdn.com/ajax/knockout/knockout-2.1.0.js'&gt;&lt;/script&gt;
  
  
  &lt;meta http-equiv="Content-Type" content="text/html; charset=utf-8" /&gt;
  &lt;title&gt;Dokument bez tytułu&lt;/title&gt;
  &lt;/head&gt;
  
  &lt;body&gt;
  &lt;p&gt;Send me spam: &lt;input type="checkbox" data-bind="checked: wantsSpam" /&gt;&lt;/p&gt;
  &lt;div data-bind="visible: wantsSpam"&gt;
  Preferred flavors of spam:
  &lt;div&gt;&lt;input type="checkbox" value="cherry" data-bind="checked: spamFlavors" /&gt; Cherry&lt;/div&gt;
  &lt;div&gt;&lt;input type="checkbox" value="almond" data-bind="checked: spamFlavors" /&gt; Almond&lt;/div&gt;
  &lt;div&gt;&lt;input type="checkbox" value="msg" data-bind="checked: spamFlavors" /&gt; Monosodium Glutamate&lt;/div&gt;
  &lt;/div&gt;
  
  &lt;script type="text/javascript"&gt;
  var viewModel = {
      wantsSpam: ko.observable(true),
      spamFlavors: ko.observableArray(["cherry","almond"]) // Initially checks the Cherry and Almond checkboxes
  };
  
  // ... then later ...
  viewModel.spamFlavors.push("msg"); // Now additionally checks the Monosodium Glutamate  checkbox
  &lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>