Fix addKeyProp to not mutate props object

[cknitt]

I just noticed that the existing implementation of addKeyProp is mutating the props, just like the

(props->Obj.magic)["key"] = key

idea that we discussed previously.

This is because Object.assign mutates its first argument.
This PR fixes that.

[cknitt]

BTW, why are we keeping the helper functions needed by the PPX in the main React module and not putting them in a separate module ReactPPXSupport or something like that?

[mununki]

I didn't know this tip adding an empty object to avoid mutating. Thanks for this tip!