title: Post parameter is always null
body: <p>Since upgrading to RC for WebAPI I'm having some real odd issue when calling POST on my WebAPI.
  I've even gone back to the basic version generated on new project. So:</p>
  
  <pre><code>public void Post(string value)
  {
  }
  </code></pre>
  
  <p>and calling from Fiddler:</p>
  
  <pre><code>Header:
  User-Agent: Fiddler
  Host: localhost:60725
  Content-Type: application/json
  Content-Length: 29
  
  Body:
  {
      "value": "test"
  }
  </code></pre>
  
  <p>When I debug, the string "value" is never being assigned to. It's just always NULL.
  Anyone having this issue?</p>
  
  <p>(I first saw the issue with a more complex type)</p>
  
  <p>The problem is not only bound to ASP.NET MVC 4, the same problem occurs for a fresh ASP.NET MVC 3 project after RC installation</p>