Clarify behavior wrt If-Match header on a GET request

Section 9.1.6 of protocol incorrectly implies that a request containing an If-Match header should return a 304 Not Modified in the case the ETag specified in the If-Match header matches, which is incorrect.
Clarify that if the If-Match header is used on a GET request and the content has changed that the server MUST return a 412 Precondition Failed and otherwise process the request as normal.