Adding a test demonstrating issue #1840.

[airdrummingfool]

This test demonstrates #1840: using unset to remove a child object leaves behind an UnsetOp in the child object's place when there is a non-trivial beforeSave on the parent object.

[flovilmart]

Awesome! Any clue for a fix proposal?

[airdrummingfool]

No ideas yet - if I get some time in the next couple of days I'll do some digging.

[airdrummingfool]

@flovilmart I found another bug that is related, but is exposed using set('key', undefined) instead of unset('key') and has a different outcome. Is it more appropriate to add it to this existing test, or write a new test?
EDIT: went ahead and added it as a separate test in #1943.

[flovilmart]

Closing as we have a fix with Parse-JS 1.9