title: "Value cannot be null" when passing null arguments to SignalR client-side proxy
body: <p>Using SignalR (v2.0.2) I'm getting this exception:</p>
  
  <pre><code>Value cannot be null.
  Parameter name: o
     at Newtonsoft.Json.Utilities.ValidationUtils.ArgumentNotNull(Object value, String parameterName)
     at Newtonsoft.Json.Linq.JToken.FromObjectInternal(Object o, JsonSerializer jsonSerializer)
     at Newtonsoft.Json.Linq.JToken.FromObject(Object o, JsonSerializer jsonSerializer)
     at Microsoft.AspNet.SignalR.Client.Hubs.HubProxy.Invoke[T](String method, Object[] args)
  </code></pre>
  
  <p>I'm calling the HubProxy with reflection, and I am passing in some null arguments that are triggering this error (aka, some items in the <code>Object[] args</code> are null). However, passing null for an argument to a method is a perfectly legitimate thing to do. Is this a bug in SignalR in that it is trying to serialize null parameters or is it a bug in Json.NET (v6.0.1) in that it shouldn't attempt to serialize a null object?</p>