[PHP-888] DBRef refactoring broke BC for $id parameter handling

See: https://jira.mongodb.org/browse/PHP-888

This restores DBRef behavior (for MongoDBRef and the collection/db helper methods) from the 1.4.x series, which was changed by 926d08d6965e88a07e2c9c0cd029484142a79c4e in #473.

Tests have been added for this legacy behavior, which may be changed should we decide to alter behavior for 1.5.0 or a future release.