CDRIVER-457 allow dbref's in bson_validate

when bson_validate is provided BSON_VALIDATE_DOLLAR_KEYS, correctly
handle the dbref style sub doc.

I.e. validate:

    {
        sub_doc : {
            $ref  : "string",
            $id   : "anything",
            $db   : "optional string",
            extra : "stuff"
        }
    }
Does BSON_VALIDATE_PHASE_NOT_DBREF indicate that we're done looking for a dbref document (ie we have found a valid one) or that this cannot be a valid dbref document?