…is canceled

In the previous implementation the `connect_to` future was spawned to let it finish when the checkout future won the race. However, in the event of the user dropping the `ResponseFuture`, the `connect_to` future was canceled as well, and all the http2 requests waiting for the connection would error as a result. This commit prevents that by spawning the `connect_to` future invariably (once it starts) thus covering both cases.

Fixes hyperium#3199

…is_inherited_by_next_request

…n task