Bluetooth: mesh: Reset Heartbeat Pub timer before sending message

The timer should be reset before sending to ensure correct
publication period.

This patch allows to pass MESH/NODE/CFG/HBP/BV-02-C.

This patch is ported from mynewt:
Commit: d4b84638df47e7ea21629e6919f547f5dcd47285

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>