Bug: LazyReferenceField passthrough on classes with methods do not work

[ErikBrendel]

I have a LazyReferenceField to a document class that I want to call methods on, but that does not work, because the __getattr__ method of a LazyReference is implemented via the subscript operator instead of a getattr call.

Relevant line: https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/base/datastructures.py#L468

Replacing return document[name] with return getattr(document, name) would solve the problem.

[bagerard]

Please open a PR and don't forget to add test cases