title: Modifying MVC form collection parameters throwing NotSupportedException
body: <p>We have a third party control that is posting the same form parameters twice. We would like to patch the problem for now and notify them so we can continue using their controls. How do you modify a form parameter that is posted to an MVC controller's action method?</p>
  
  <pre><code>public ActionResult Index()
  {
      var httpParams = this.HttpContext.Request.Params;
      if (httpParams["form_element"] != null ) {
          httpParams["form_element"] = "some new value"; // NotSupportedException: Collection is read-only
      }
      return new CustomActionResult(this);
  }
  </code></pre>
  
  <p>Trying to alter the request parameters does not work - it throws a <code>NotSupportedException</code>. Is there a way to redirect this request or do we need some custom HTTPHandler/Module to support this change? We want to control the request body values before the response is processed.</p>