res/res_pjsip_nat: Ignore REGISTER requests when looking for a Record-Route

We will only rewrite the Contact header if there is no Record-Route header in
the received request. If a malfunctioning proxy places a Record-Route header
into a REGISTER request, we will decide that we shouldn't update the IP/port
in the Contact header, and we will end up storing a contact with an AoR that
contains the NAT'd IP address.

While it is nice to have the proxy *not* send a Record-Route in a REGISTER
request, it's also a good idea to not process the header in a non-dialog
message. This patch updates the code to explicitly ignore the Record-Route
header in REGISTER requests.

ASTERISK-25387 #close

Change-Id: I4bd3bcccc4003d460cc354d986b0dea2e433ef3f