res_pjsip_outbound_authenticator: Increase CSeq on authed requests.

The way PJSIP generates an authenticated request is to use a previous
request as a template. This means that the authenticated request will
have the same Call-ID, From header (including tag), and CSeq as the
original request. PJSIP generates a new branch on the Via header to
indicate that this is a new transaction, though.

There are some SIP implementations, though, that do not notice the
change in the branch and therefore will match the authed request to the
original request's transaction. Since the CSeq is the same, the server
will repeat the response it sent to the original request.

This patch aids interoperability by increasing the CSeq of the authed
request by one.

ASTERISK-24845 #close
Reported by: Carl Fortin
Tested by: Carl Fortin

Change-Id: I39c4ca52e688a9f83bcc1878371334becdc5be01