Allow multiple a=msid attributes

[ibc]

Details

As per RFC 8830, a media section can contain multiple a=msid attributes. This is because the MediaStreamTrack being sent in that media section may belong to múltiple MediaStream. And hence, the msid field must be an array.

Also per spec, msid value consists on a mandatory id string (which can be an id or '-' string) and an optional appdata. Within the WebRTC context, id is the id of the MediaStream while appdata is the id of the MediaStreamTrack.

Also adapt the tests.

[clux]

sounds sensible to me.

checks out from the rfc

There may be multiple "msid" attributes in a single media description.

[clux]

for purposes of semver, do you think this is a bugfix or a breaking change? :D

[ibc]

for purposes of semver, do you think this is a bugfix or a breaking change? :D

I'd say this is a breaking change since any previous write usage of media[0].msid = "11111 22222" would now lead to bad results.

[clux]

released in 3.0.0

[ibc]

Thanks a lot @clux. Next I will create a PR to update types/sdp-transform. Said that, would you accept a PR that migrates sdp-transform to TypeScript (of course the NPM published library would be transpired JS)? Another option would be to include a sdp-transform.d.ts in the repo but I find it not as reliable and more error prone than directly switching to TS.

[ibc]

I'm already doing it... #105 (WIP)