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
since pools can keep multiple persistent connections which may have been terminated by the peer already, exhausting the one retry attempt from the persistent plugin may make request fail before trying it on an actual connection. in this patch, requests which are preceded by a PING frame used for probing are therefore marked as such, and do not decrement the attempts counter when failing
