The request.spec GUC contains the schema cache structure in json.
It's only available when the root endpoint(/) is requested and when
db-root-spec is not empty.
Also correct db-root-spec to accept a schema.
Allows using header "Prefer=headers-only" explicitly to get a response with only a Location header.
BREAKING CHANGE: Change default for POST request from headers-only to minimal
Resolves#1656