Fix: Remove props.key even when key === 0 or key === ""

[jviide]

This pull request fixes a case where a component may have props.key set when key === 0 or key === "".

Added tests for props.key removal (as well as props.ref removal as that code got also changed a bit).

As luck would have it this also cuts down preact.js.gz size by 1 byte :)

[coveralls]

Coverage remained the same at 100.0% when pulling 96dbf6a on jviide:fix-props-key into 0eb7a5d on developit:master.

[JoviDeCroock]

Nice find, going strong! 💪

[marvinhagemeister]

Sweet! 👍