MT#6475 check method (ACK or PRACK) before deleting transaction

commit 22b8f00398f7fcffc291063c19dae46a11f41227
Author: Raphael Coeffic <rco@iptel.org>
Date:   Tue Apr 15 14:49:49 2014 +0200

    sip: b/f: check method (ACK or PRACK) before deleting transaction

    In case PRACK is received after 200, the transaction would be removed without checking the method.

(cherry picked from commit 99ca300e96924c46a3757dcc6b98be15896ca8fb)