Get the issuer certificate from a 'Link' response header
Param: linkHeader - ResponseHeader Line of the form Link: <https://acme-v01.api.letsencrypt.org/acme/issuer-cert>;rel="up"
Pem-encoded issuer certificate string
See Implementation
Get the issuer certificate from a 'Link' response header
Param: linkHeader - ResponseHeader Line of the form Link: <https://acme-v01.api.letsencrypt.org/acme/issuer-cert>;rel="up"