Implement integration tests

[pathob]

In order to be confident that all our endpoints are really working, we need integration tests for all endpoints and their sub-endpoints.

The kind of tests that we need in most cases:

• get data
• set / add data with all attributes set in request bean
• set / add data with all attributes set in request bean, then set ("override" existing) data with no attributes set in request bean, then check bean is still the same (refers to Adjust API behavior regarding unset model attributes #92)
• delete data

The kind of tests that we DON'T need in every case:

• check authentication - use a single authentication test class instead for a common endpoint (e.g. /settings) and use unit-tests instead to make sure that @ResourceFilters annotation is present with appropriate resource filter.