sync_once hangs for minutes when server offline

latest release as well as master.

`sync_once` is called with a timeout of 60 seconds.

during sync the server on localhost was shut down. but after the client was stuck for minutes - more than the 60 seconds - before finally timing out.

seems also that with certain other 404 responses the client seems to hang until the timeout period(?).

you may want to set both a connect timeout and a read timeout separately.