Allow choosing WebRequest stack for HttpClient

Trying to fix a bug where Silverlight client starts the connection only using ClientHttp. With this bugfix Silverlight client get possibility to start the connection using BrowserHttp.

Silverlight client can do this by instantiating DefaultHttpClient and injecting System.Net.Browser.BrowserHttp into the constructor. The default(empty constructor) will automatically use System.Net.Browser.ClientHttp.