Ability to customize the JsonSerializer for a client connection

- Connection class has JsonSerializer property
    - ConnectionExtensions has methods for serializing and deserializing using the serializer set on the connection
    - HubProxy tests provides a default serializer for IConnection.JsonSerializer {get; }
    - Added the test VerifyThatChangingTheJsonSerializerWorks

Issue: https://github.com/SignalR/SignalR/issues/1373