Harware appliances from Kemp seem to validate the contents of the
`meta` object, even if none of the properties in the `meta` object
is set. According to the RFC, the `meta` object, as well as its
properties are optional, so technically this should be fixed by
the manufacturer.
This commit is to see if we validation of the `meta` object is
skipped if it's not available in the response.
Refactored some of the existing bits for testing the Authority
API by creation of a new LinkedAuthority interface and changing
visibility of the MockAuthority to be usable by other packages.
At this time, not all of the functions of MockAuthority it usable
yet. Will refactor when needed or requested.
When constructing links we want to read the required host and protocol
information in a dynamic manner from the request for constructing ACME
links such as the directory information. This way, if the server is
running behind a proxy, and we don't know what the exposed URL should
be at runtime, we can construct the required information from the
host, tls and X-Forwarded-Proto fields in the HTTP request.
Inspired by the LetsEncrypt Boulder project (web/relative.go).
