title: knockout.js observable is not updating
body: <p>Team,</p>
  
  <h2>I have a very simple html page with a viewmodel as follows.</h2>
  
  <pre><code>@{
      Layout = null;
  }
  &lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
      &lt;script src='E:\Trials\ClientSide\MyTrials\knockout-2.2.1.js' type='text/javascript'&gt;&lt;/script&gt;
      &lt;script src='E:\Trials\ClientSide\MyTrials\jquery-2.0.0.js' type='text/javascript'&gt;&lt;/script&gt;
      &lt;title&gt;Index&lt;/title&gt;
      &lt;script type="text/javascript"&gt;
  
          function PersonViewModel()
          {
              firstName = ko.observable("FirstN")
          };
  
          $(document).ready(function () {
              var person = new PersonViewModel();
              ko.applyBindings(person);
          });
  
      &lt;/script&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;div&gt;
          &lt;h3&gt;Details&lt;/h3&gt;
          &lt;p&gt;First Name: &lt;input data-bind="value:  firstName()" /&gt;&lt;/p&gt;
          &lt;p&gt;First Name From span: &lt;span data-bind="text: firstName()" &gt;&lt;/span&gt; &lt;/p&gt;
      &lt;/div&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <hr>
  
  <p>Its very simple and self explanatory. script block contains a view model, and on doc ready function, binding happens. And the html is also simple enough. One input and one span bound to same property firstName which is observable. 
  But the pain is when I change the value from the input, the span does not update. What am I missing?
  Regards
  Vivek</p>