DTMF hooks: Leaving channels need to push any collected digits into the bridge.

Any partially collected DTMF digits for a DTMF hook need to be pushed into
the bridge when a channel leaves the bridging system as if there were a
timeout.

Review: https://reviewboard.asterisk.org/r/4199/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@428601 f38db490-d61c-443f-a65b-d21fe96a405b