Verifying enum arugment errors
Adding Repository tests

Some of the methods were being tested as part of jobs; This is to
verify their behavior
Specing out update_header_with_doi_predicate!
Enumerating AdditionalAttribute#key

For now, we have a limited number of predicates; Lets see how this
holds out.
Adding spec for .fieldset_for
Adding additional documentation and admonitions

[skip ci]
Appeasing the rubocop