Group command needs to make a copy of the obj it passes to JS

GroupStage::processObject needs to make a copy of obj before passing it down into js, because those objects can be copied into the accumulating object (after which that object won't be readable).

It doesn't appear that this is a crasher (because we use the scope's generation to trap the invalid access), but it does mean that valid operations will fail with uasserts later on.

We should make obj an owned copy and add a test for the functionality