org-export: Allow "string with spaces" as #+OPTIONS: values

* lisp/ox.el (org-export--parse-option-keyword): Allow `read'ing the
option value as far as needed.  Do not restrict the value to the first
whitespace only.  This way, one can use Elisp strings with all the
escaping options as values.
* testing/lisp/test-ox.el (test-org-export/parse-option-keyword): Add
test for option value with a space inside.

Reported-by: Pierre Balayé <pierrebalaye@gmail.com>
Link: https://list.orgmode.org/orgmode/CANpQAF-n+4xhNvL8aaP8j2gJ70vbu80wmh9a4Oj0BxNHA5-yDA@mail.gmail.com/

lisp/ox.el

@@ -1404,14 +1404,15 @@ Optional argument BACKEND is an export back-end, as returned by,
 e.g., `org-export-create-backend'.  It specifies which back-end
 specific items to read, if any."
   (let ((line
-	 (let ((s 0) alist)
-	   (while (string-match "\\(.+?\\):\\((.*?)\\|\\S-+\\)?[ \t]*" options s)
-	     (setq s (match-end 0))
-	     (let ((value (match-string 2 options)))
-               (when value
-                 (push (cons (match-string 1 options)
-                             (read value))
-		       alist))))
+	 (let (value alist)
+           (with-temp-buffer
+             (insert options)
+             (goto-char (point-min))
+             (while (re-search-forward "\\s-*\\(.+?\\):" nil t)
+               (when (looking-at-p "\\S-")
+                 (push (cons (match-string 1)
+                             (read (current-buffer))) ; moves point
+                       alist))))
 	   alist))
 	;; Priority is given to back-end specific options.
 	(all (append (org-export-get-all-options backend)

testing/lisp/test-ox.el

@@ -198,7 +198,10 @@ num:2 <:active")))
   (should
    (let ((options (org-export--parse-option-keyword "H: num:t")))
      (and (not (plist-get options :headline-levels))
-          (plist-get options :section-numbers)))))
+          (plist-get options :section-numbers))))
+  ;; Parse spaces inside brackets.
+  (let ((options (org-export--parse-option-keyword "html-postamble:\"test space\"" 'html)))
+    (should (equal "test space" (plist-get options :html-postamble)))))
 
 (ert-deftest test-org-export/get-inbuffer-options ()
   "Test reading all standard export keywords."