use await GetRequestStreamAsync on System.Net.Http.HttpClientHandler SendAsync to avoid blocking

I've been using the System.Net.Http Async methods with the monotouch 6.3.1 preview release, and when a server is down (in our case an aws ec2 spot instance with an elastic ip terminates) the  async request completely blocks the UI... after a little investigation (using a custom httpclienthandler), the problem lies in the var stream = wrequest.GetRequestStream() call that blocks execution... changing it to var stream = await wrequest.GetRequestStreamAsync (); resolves the problem.

Hope it helps!