Currently, if the remote closes the connection at the same time that the
pool selects it to use for a new request, the connection may actually
hang. This fix will now more allow the keep-alive read to check the
socket even when the `Conn` think it's busy.
If the connection was closed before the request write happened, returns
back an `Error::Cancel`, letting the user know they could safely retry
it.
Closes#1439
