dont mutate event type name, just pass it through

[uxtx]

This resolves an issue with mutating the type string that was causing some third party test failures

Closes #7418

[ghost]

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[quantizor]

keyDown isn't a valid native event name, the original behavior is correct and the other implementations are wrong.

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[zpao]

I appreciate the quick issue filing & PR to address, however as noted it's not actually a problem in React. I commented in #7418 (comment) with more details.