RFC8040 compliance: The RESTCONF server MUST support the HEAD method

https://tools.ietf.org/html/rfc8040#section-4.2
The RESTCONF server MUST support the HEAD method. The HEAD method is
sent by the client to retrieve just the header fields (which contain
the metadata for a resource) that would be returned for the
comparable GET method, without the response message-body.
The current behavior is not consistent with the spec

> curl -u admin:admin -I http://localhost:8181/rests/data/toaster:toaster
< HTTP/1.1 200 OK
< Set-Cookie: JSESSIONID=node0tp7ksek0vyu11fvjctgrgqwlw67.node0; Path=/; HttpOnly
< Expires: Thu, 01 Jan 1970 00:00:00 GMT
< Set-Cookie: rememberMe=deleteMe; Path=/; Max-Age=0; Expires=Sun, 01-Nov-2020 22:09:23 GMT
< ETag: "2009-11-20-toaster"
< Last-Modified: 2020-Nov-02 22:09:23
< Content-Type: application/yang-data+json
< Content-Length: 126