Gen-ART 26

From: Mary Barnes

Description: -Section 5.3.3.1 [6.3.3.1]:
--1st para: "a request returns" -> "a request MUST return" 
--1st para: "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"
--2nd para: "message code is set" ->  "message code MUST be set"

Notes: Will fix text as suggested.