Changes

1) Decision to divert data to other thread should only be made in within OnStartRequest().

There seemed to be general agreement that the decision to offload OnDataAvailable() to a different thread should only be made possible within OnStartRequest(). By this point, we will have the final channel for the request (i.e. any redirects, auth requests, etc, will have been resolved). We will also have the Content-type of the request, which we will need to determine whether the data should be handled by an off-thread consumer (like the HTML parser) in the first place.