persistent plugin: drop , allow retries for ping requests, regardless of idempotency property

the previous option was there to allow reconnecting on non-idempotent (i.e. POST) requests, but had the unfortunate side-effect of allowing retries for failures (i.e. timeouts) which had nothing to do with a failed connection error; this mitigates it by enabling retries for ping-aware requests, i.e. if there is an error during PING, always retry.

Closes https://github.com/HoneyryderChuck/httpx/issues/82#issuecomment-2797755524