fix: iceStopWaitingOnServerReflexive is resetting the peerConnectionDelegate

[iomariani]

When enabling the iceStopWaitingOnServerReflexive configuration and you have a custom sessionDescriptionHandler with custom peerConnectionDelegate, it gets erased.

In the code the delegate is rewritten to the one defined, without using the original delegates.

This fix simply merges the previous peerConnectionDelegate, if exists, to the new peerConnectionDelegate.

https://github.com/onsip/SIP.js/blob/e9ae99c1c92a7ec1e6fdac6c2ff685d45c2b0f80/src/platform/web/session-manager/session-manager.ts#L573-L576