The existing guideline recommends to do a GET on a tag to check its
existence, while our HTTP guideline recommends HEAD for such checks.
Since this is just a check, we should recommend HEAD in this case.
Wording was also added to the HTTP guideline stating that in all cases
where HEAD is implemented, the corresponding GET must be implemented,
too.
Closes-Bug #1677360
Change-Id: I36045e84e906dfbdf60c8989e2a5a5fb39b7cc34