Conformance to HAP Specification section 5.7

[tgu]

In the process of finding some bugs in my application code I have found some possible changes.

• Value should only and always be present for read permission
• Sometimes the response should be 207 multi-status with an HAP Status Code
• More diverse response messages for write request

[tgu]

I think I can provide some tests.

[Bouke]

Great work! I've merged your PR in d7ce5f498fac70792af5a8b1468d49281b26b6a2.