Add comments to clarify change to www-authenticate test

Trivial patch to tidy-up change to the functional test for
www-authenticate header and add a comment to explain
that multiple header values might be returned.

Change-Id: If62cb3fd9e11450a2be0cec71e80ecb74a959d04
Related-bug: 1368048
Merge "Add comments to clarify change to www-authenticate test"