unreachable panic triggered if using client::conn with the Version set to HTTP_2

While the `Server` and `Client` will both coerce `Version::HTTP_2` to `HTTP_11`, the `client::conn::SendRequest` doesn't do that. Thus, [this panic](https://github.com/hyperium/hyper/blob/master/src/proto/h1/role.rs#L670) is triggered.

It seems best to just add a case for it just like there is in the [server encode path](https://github.com/hyperium/hyper/blob/master/src/proto/h1/role.rs#L270).