Reference hydration assumes document exists

[Bilge]

When hydrating referenced documents the current logic assumes that an identifier will be passed and that a document with that identifier already exists. This may not be the case, however, and in the event that no identifier is passed it should be possible to instantiate a new document in lieu of an existing one.

[veewee]

This can be fixed by adding a custom strategy. Personally I have never encountered a situation where I wanted to create a referenced resource at the same time as attaching it to another entity.
If this is something that is required frequently, it can be added as a default strategy to this repository.

[veewee]

The failing tests are due to:
#20