[winmd servicebus] custom message properties as date throws InvalidCastException

[deepakverma]

Winmd code: var messagesend = BrokeredMessageSettings.CreateFromText("test/xml","this is my message");
messagesend.Properties["orderid"] = new DateTime();
sb.SendMessageAsync("test", messagesend).AsTask().Wait();

invalid cast is thrown by
https://github.com/WindowsAzure/azure-sdk-for-net/blob/dev/microsoft-azure-servicelayer/Microsoft.WindowsAzure.ServiceLayer/ServiceBus/CustomPropertiesDictionary.cs
datetime cast needs to be added to the encode and decode function