title: Call to a specific Client (using Invoke) fails in SignalR - conversion issue
body: <p>Can anyone explain me why the second call (to an individual client) is invalid?</p>
  
  <pre><code>IHubContext hubContext = GlobalHost.ConnectionManager.GetHubContext&lt;SymHubGrid&gt;();
  
  ((ClientProxy)(hubContext.Clients.All)).Invoke(method, msg);   // OK
  
  ((ClientProxy)(hubContext.Clients.Client(hubConnID))).Invoke(method, msg);  // FAIL
  </code></pre>
  
  <p>It fails with the following exception:<br>
  {Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: Cannot convert type 'Microsoft.AspNet.SignalR.Hubs.ConnectionIdProxy' to 'Microsoft.AspNet.SignalR.Hubs.ClientProxy</p>
  
  <p>It is connected to my earlier <a href="https://stackoverflow.com/questions/16079813/how-to-use-a-variable-as-a-method-name-using-dynamic-objects">post</a>.</p>
  
  <p>Thanks</p>