Fix shouldUpdateReactComponent not considering props.key

**Apparently @spicyj already has a pending PR for this that should be considered instead, https://github.com/facebook/react/pull/591.**

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`key` is currently only obeyed by descendants of the root-node, the root-node itself does not obey `key`. This simple fix ensures that it works on the root-node as well.

I would consider this a bug and speaking hastily to petehunt, he seemed to agree:

`<@petehunt> yeah i think that would be a bug`

My personal motivation for this is my `<swf>`-component that I will be releasing, it requires this to support changing `src` at run-time (although other workarounds do exist). In my opinion, there's no reason why the root-node should be any different from the other children in this regard.