Document that Copy::$copy can be set to null
Test Copy@copy can be set to Null
Article doesn't need to ensure $copy can be set to Null.
Make existing tests unit tests.
Remove unneeded method
Add unit test for the abstract entities
Prepare Axstrad/Content for release
Merge branch 'release/axstrad-content' into develop
Merge branch 'develop' into release/doctrine-extensions
Prepare Axstrad/DoctrineExtensions for release
Merge branch 'release/doctrine-extensions' into develop