test: Improve test coverage of serializers (#439)

* test: Improve test coverage of serializers.authors

* test: Remove unnecessary import

* test: Improve test coverage of serializers.authors (cont)

* test: Improve test coverage of serializers.reference

* test: test: Improve test coverage of serializers.anchor

* test: Improve test coverage of serializers.series

* test: Check if None instead of raising AssertionError when it is

* test: Remove trailing full stop from initials

* test: Add docstrings to tests

* test: Add URI scheme