macro: Properly collect multiple authors

* lisp/org-macro.el (org-macro--collect-macros): Multiple lines are
allowed for AUTHOR keyword (per `org-export-options-alist') so make
sure we collect all of them.
* testing/lisp/test-org-macro.el (test-org-macro/author): Add test.

Reported-by: Vinicius Vinicius <vinicius.vin@yandex.com>
<http://lists.gnu.org/r/emacs-orgmode/2021-10/msg00727.html>

lisp/org-macro.el

@@ -125,7 +125,7 @@ previous one, unless VALUE is nil.  Return the updated list."
   "Collect macro definitions in current buffer and setup files.
 Return an alist containing all macro templates found."
   (let ((templates
-         `(("author" . ,(org-macro--find-keyword-value "AUTHOR"))
+         `(("author" . ,(org-macro--find-keyword-value "AUTHOR" t))
 	   ("email" . ,(org-macro--find-keyword-value "EMAIL"))
 	   ("title" . ,(org-macro--find-keyword-value "TITLE" t))
 	   ("date" . ,(org-macro--find-date)))))

testing/lisp/test-org-macro.el

@@ -314,6 +314,15 @@
 	    (org-macro-replace-all org-macro-templates)
 	    (buffer-substring-no-properties
 	     (line-beginning-position) (point-max)))))
+  ;; Return AUTHOR keyword value.
+  (should
+   (equal "author 1 author 2"
+	  (org-test-with-temp-text
+              "#+author: author 1\n#+author: author 2\n<point>{{{author}}}"
+	    (org-macro-initialize-templates)
+	    (org-macro-replace-all org-macro-templates)
+	    (buffer-substring-no-properties
+	     (line-beginning-position) (point-max)))))
   ;; When AUTHOR keyword is missing, return the empty string.
   (should
    (equal ""