Serialize the id if its an ObjectID, fixes #5

[myartsev]

I took a stab at fixing #5

Note: I imagine that the other fields in a mongo DB could also be ObjectIDs.
Should those be serialized as well? I don't want to open a can of worms in this PR,
so here I fix the ID field which is a very common scenario.
We can open another issue regarding other fields.

[myartsev]

TypeError: clone[primaryKey].toHexString is not a function
That's strange. Apparently ObjectID.isValid(1) returns true...

Looks like a bug with the MongoDB driver, I'll check with them.
In the meantime I'll change how to determine if this field is an ObjectID.
Apologies for all the noise, should have ran the tests locally.

[myartsev]

Retracting pull request.
While this works for find, it breaks down in other areas; relationships, updates & deletes.

Need a more complete solution to support Object IDs across the board.