Set last_packet_sent_at only if the packet was actually sent.  Indicate in the return value (currently not used in any caller) if we've actually not sent the packet.
Set last_packet_sent_at only if the packet was actually sent.  Indicate in
the return value (currently not used in any caller) if we've actually not
sent the packet.

RubyDevInc/paho.mqtt.ruby,RubyDevInc/paho.mqtt.ruby