MutableOutlet unit test has a bug

[darvelo]

I think the mock setter below should have html as an argument that it uses to set the internal _innerHTML property that's tested later on:

https://github.com/darvelo/ether/blob/master/test/unit/classes/mutable-outlet.spec.js#L38

[darvelo]

Actually, it works as intended (to be pretty much a no-op test in a Node environment), but making this change would at least ensure the html string passed into the outlet constructor is used on the setter.