Correcting the response to /call/hangup

Functional changes:

- Both receptionists and administrators are allowed access to the interface.
- The content of the response in case of a successful operation has been corrected to match [the specification](https://github.com/AdaHeads/Call-Flow-Control/wiki/Protocol-Call-Hangup).

Non-functional changes:

- Fixing misspelled identifiers in `package Response`.
- Source text layout.
- Reference to issue #146.
- Reference to the interface specification.