As I was tinkering with an aggregate pipeline I found while looking for different ways of getting all properties within a collection, I noticed my tests were failing because mongomock doesn't support $objectToArray yet.

I tried to implement the function, but I wasn't 100% sure where to put it and how to properly test it.
So please, see this more of a draft or proposal. I will work on it further, but if you like my approach and can nudge me in the right direction, I'd be glad to contribute!

Oh, also, I didn't branch off the latest develop but of the last commit that was not failing, hope this is alright!