[Performance Improvement] differ between GET jwks_url to fetch keys and HEAD jwks_url to check whether zone is accepted

[nenaraab]

We need to check whether HEAD of jwks_url checks the x-zone_uuid as well.

PRO:

• no (un-)marshal json body

CONTRA:

• increase code complexity
• erroneous zoneId needs to be handled / memorized in both cases GET and HEAD