Draft: utils: Keep ref on session

Otherwise session becomes undefined when we leave this function and soup_session_send_async is still in progress which doesn't end well.

@evangelos.tzaras could you check if this helps with #24 (comment 271992) ? It obviously leaks but I'd be interested if it fixes the assertion for you (or at least gives us a valid address in the assertion (in which case we know exactly what triggers the assertion)).