Adjust API behavior regarding unset model attributes

[pathob]

Following the discussion in aservo/confapi-crowd-plugin#16 (comment) and in Slack, we need to decide for a strategy of how to handle unset model attributes in REST API requests.

[pathob]

Usually, I wouldn't do a review because both PR's are still work-in-progress

[pathob]

Linked to:

• Make API behaviour more consistant confapi-jira-plugin#42
• Make API behaviour more consistant confapi-confluence-plugin#77