msid parsing is broken for Firefox SDPs from 3.0.0

[thatsmydoing]

#104 breaks parsing of Firefox SDPs, a typical msid of

a=msid:- {774d17bb-e827-4022-83d2-76676579be59}

is parsed as only { id: '-' } so when it gets reserialized, it becomes:

a=msid:- undefined

I think this is related to #56 since both values there are actually tokens but still use the \w- regex. Though I'm a bit surprised that the parsing failed silently and serialization when no appdata is present is wrong.

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For reference: in our use case, we use sdp-transform to modify the offer SDP. This results in an SDP where there are duplicate msid lines since only the appdata part is unique and that was getting dropped. When Chrome receives the offer, it rejects these SDPs with a Duplicate a=msid lines detected error.