Remove the HttpRequest::get() method.

I'm in the process of adding support for cancelling HTTP requests, and I need to really make sure no one is messing around with the evhttp_request, otherwise we're all going to lose our marbles.
Could all of these Host header setting just be done in HttpConnection::MakeRequest() [you could presumably check if there's one already set if you think you might actually want to set something which isn't part of the URI] ?