RTP event payload type can be set incorrectly when sending DTMF on an incoming call

[CNH01]

Suggested PR:
#1165

Scenario:

• SIP transport receives an INVITE request with SDP containing the capability 96 telephone-event 0-15
• Invite is accepted (by creating a SIPUserAgent and calling AcceptCall(SIPRequest inviteRequest))

Result:
The SIPUserAgent's media session's local track's capabilities are changed to support
96 telephone-event 0-15
rather than
101 telephone-event 0-16

So the SDP in the 200 OK when answering the call contains 96 telephone-event 0-15.

This becomes an issue when calling SendDtmf(byte tone) on the SIPUserAgent, as this calls SendDtmf(byte key, CancellationToken ct) on the AudioStream which, in turn, defaults to using 101 telephone-event 0-16 (RTPSession.DTMF_EVENT_PAYLOAD_ID).

So we end up sending DTMF with payload type 101, despite advertising that we will send payload type 96 when answering the call.

[sipsorcery]

#1165 merged.