Adding a test demonstrating issue #1942.

[airdrummingfool]

This test demonstrates #1942: saving an object after calling set('key', undefined) does not result in key being undefined in object's beforeSave. I originally thought this was Cloud Code only, but that is not the case.

[flovilmart]

This one seems to be a JS-SDK behaviour. When you do set('key', undefined), the JS-SDK registers it as a no-op as this is equivalent/equal to calling set('key')