Problematic `replace` with composite pk without _id

[pmaselkowski]

Mongo will anyway store _id, and as with composite PK without _id field it will simply insert new document each time. #59 Might be remedy, need to add pk's index definition to PrimaryKey annotation too.

[pmaselkowski]

Workaround - use upsert

[pmaselkowski]

Maybe it should remove _id field if set externally and not present in model

[pmaselkowski]

Example exception message when upserting model with different pk:

The _id field cannot be changed from {_id: ObjectId('5a670804a3d24b45714fb6f4')} to {_id: ObjectId('5a670804a3d24b45714fb6f5')}.