Make MongoCollectionReader's iter be able to yield raw docs

[thorwhalen]

In MongoCollectionReader's init we assert that iter_projection can't be None (before that we do iter_projection = iter_projection or (ID,) to replace the None).

But it seems it could be good to not have a projection at all sometimes, to be able to see the raw docs.

All it (seems) it would take is to remove that assertion as well as the iter_projection = iter_projection or (ID,), since mgc.find(projection=None) i valid.