Test `/operatingsystems`

Add some tests for path `/operatingsystems` and `/operatingsystems/:id`. One of the test methods fail; a bug report should be written, and this is on my to-do list. Make some changes to entity `OperatingSystem` and field `IntegerField` to facilitate the proper functioning of the new tests, and add tests for the updated `IntegerField`.

Update some docstrings. Reduce the default max length of field `StringField` to make debugging output easier to read.