ghub-response-link-relations: Fix for GitLab

They changed the name of the header from `Link' to `link', or maybe
they always did the latter (though I expect this issue would have been
reported a long time ago, if that were the case).

To this day, the section [1] where they documents this header, is name
"Pagination `Link' header" (with `Link' using code syntax), which is
misleading, especially consider that HTTP headers traditionally are
capitalized [2], but in their API it is not (while some others are).

[1]: https://docs.gitlab.com/ee/api/rest/index.html#pagination-link-header
[2]: https://www.iana.org/assignments/http-fields/http-fields.xhtml#field-names

lisp/ghub.el

@@ -447,7 +447,9 @@ this function is called with nil for PAYLOAD."
               (setq-default ghub-response-headers headers))
             page)
         (cdr (assq 'link-alist ghub-response-headers)))
-    (and-let* ((rels (cdr (assoc "Link" (or headers ghub-response-headers)))))
+    (and-let* ((headers (or headers ghub-response-headers))
+               (rels (cdr (or (assoc "Link" headers)
+                              (assoc "link" headers)))))
       (mapcar (lambda (elt)
                 (pcase-let ((`(,url ,rel) (split-string elt "; ")))
                   (cons (intern (substring rel 5 -1))