Replace 204 response with empty map for get empty properties, fixes #83.

- This makes client-side code slightly simpler, since it removes an edge case
  entirely - just return an empty map for empty properties.
- The 204 response was undocumented, so this does not break documented
  behavior.
- Remove duplicate integration tests, replace with simple full-stack test.
Its used by an annotation processor to pull out the javadoc for documentation. However, the snippet this thing generated wasn't actually included in the documentation, so removing it has not visible effect on the manual.