gen-art-26

- "a request returns" -> "a request MUST return".
- "then the message code is the response code that matches
  the request" -> "then the message code MUST be set to the response
  code that matches the request".
- "message code is set" -> "message code MUST be set".

Closes deanwillis/p2psip-base-master#159
Merge pull request #159 from petithug/gen-art-26

Gen-ART 26 2119 questionable usage, does no harm.