Ensure Asterisk acknowledges ACKs to 4xx on Replaces errors

Ensure Asterisk acknowledges ACKs to 4xx on Replaces errors

Asterisk was not setting pendinginvite in the upper half of
handle_request_invite such that the 4xx was retransmitted repeatedly even
though an ack was received for every retransmission.

(closes issue ASTERISK-19303)
Reported by: Jon Tsiros
Patches:
  fix-19303.patch uploaded by Jeremiah Gowdy (license 6358)

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Merged revisions 358115 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@358116 65c4cc65-6c06-0410-ace0-fbb531ad65f3