Fixtures should only have data in them directly pertinent to testing.

Testing fixtures should only contain data related to the test they are used in. That means if you have a test and fixture set that do not examine header information, you shouldn't have header information in the fixtures.

This also means that if you are not examining the request XML documents then, why include that data in the fixture? Take it out.

This is naturally lower priority than getting some kind of testing around an API call if there is no testing at all around some method. But, as a matter of improving practice and improving testing this should be an exercise in increasing the effectiveness of the tests as well as reducing the amount of information in the test system to its minimum.