Replace it for correct property_exists() when the element
being inspected is a property of object/class.

Amended and squased changes:
- keep mongo unmodified. The information is array, hence correct.
- fix a couple of messaging phpdocs that were incorrect.

Amended take#2:
- As far as mongo resturns BSONDocument that is ArrayObject, aka
implements ArrayAccess, we have decided to explicitly cast results
to array so existing array_key_exists() and other accesses will
continue working the same.