don't bookkeep retry attempts when errors happen on just-checked-out open connections

in case of multiple connections to the same server, where the server may have closed all of them at the same time, a request will fail after checkout multiple times, before starting a new one where the request may succeed. this patch allows the prior attempts not to exhaust the number of possible retries on the request

it does so by marking the request as ping when the connection it's being sent to is marked as inactive; this leverages the logic of gating retries bookkeeping in such a case

Closes https://github.com/HoneyryderChuck/httpx/issues/92