Add unit tests

[adolski]

Add at least partial unit test coverage, refactoring for better testability where necessary.

[gaurijo]

Book methods w/test coverage:

• bulk_upsert
• bulk_update
• internet_archive_url
• hathitrust_handle
• uiuc_catalog_url
• as_json
• to_csv
• params_from_marcxml

(in progress):

• service
• user tests *

[adolski]

@gaurijo I've renamed this and opened #17 for the controller tests.