Renamed ReactDescriptor-test

I forgot to rename the test.
Merge pull request #2305 from sebmarkbage/missingrename

Rename ReactDescriptor followup
Use Object.assign instead of merge, mergeInto, mixInto and copyProperties

This makes it easier to contribute without having to learn a bunch of
slightly different helpers and remember their slightly different
signatures and semantics.

We'll probably start using ES7 spread properties instead of merge in the
future when at least one more transpiler supports it.