Shouldn't fire (un)mute event on already (un)muted track

[jan-ivar]

We shouldn't fire the mute event on an already muted track (track.muted == true).
We shouldn't fire the unmute event on an already unmuted track (track.muted == false).

This seems like a desirable property we should enforce in this spec. Yet we don't in the algorithm we expose, set a track's muted state (it allows firing an event regardless of existing state).

We already do so with similar algorithms like add a track and remove a track, so we probably should here as well.