While executing a trigger, we have to set thd->abort_on_warning to the value
it had at trigger creation time.


mysql-test/r/trigger.result:
  Add result for bug#6951.
mysql-test/t/trigger.test:
  Add test case for bug#6951.
sql/sp_head.cc:
  While executing a trigger, set thd->abort_on_warning to the value it had at
  trigger creation time.