Outbound calls: handle forking scenario / early media

[virgilio-a-cunha]

In outbound calls scenario that there are forking and the UAC use early media(183), the asterisk-i reply with prack and when one UAC sent final response (200 OK), this message doesn't contains SDP, so asterisk-i use the last negotiated SDP even if the negotiated SDP belongs the another UAC.

To solve this problem, the asterisk-i should not send support 100Rel to force the final response contain SDP and asterisk-i use it.