code: public Optional<Link> getLinkBy(final String rel) {
          final List<Link> links = getLinksBy(rel);
          return links.isEmpty()
                  ? Optional.empty()
                  : Optional.of(links.get(0));
      }
docs: <p>
  Returns the first (if any) link having the specified link-relation type.
  </p>
  <p>
  If CURIs are used to shorten custom link-relation types, it is possible to either use expanded link-relation types,
  or the CURI of the link-relation type. Using CURIs to retrieve links is not recommended, because it
  requires that the name of the CURI is known by clients.
  </p>
  
  @param rel the link-relation type of the retrieved link.
  @return optional link
  
  @see <a href="https://tools.ietf.org/html/draft-kelly-json-hal-08#section-8.2">draft-kelly-json-hal-08#section-8.2</a>
  @since 0.1.0