Key should not be passed as a prop

[rmorshea]

As per this answer, key is internally used and not exposed as a prop. They value of the key can be useful though, so to avoid repetition, we'll pass on the key if its listed an explicitly kwarg of the render function decorated by @component.

[rmorshea]

See commit message from #421 - we should actually raise here when key is parameter of the render function since it could cause confusion.