Make toggling checkboxes work again

* eww.el (eww-update-field): Make toggling checkboxes work
again (bug#21881).

Backport:

(cherry picked from commit 5e56f606)

lisp/net/eww.el

@@ -1177,16 +1177,19 @@ See URL `https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Input'.")
      (eww-update-field display))))
 
 (defun eww-update-field (string &optional offset)
-  (if (not offset) (setq offset 0))
+  (unless offset
+    (setq offset 0))
   (let ((properties (text-properties-at (point)))
 	(start (+ (eww-beginning-of-field) offset))
 	(current-end (1+ (eww-end-of-field)))
-	(new-end (1+ (+ (eww-beginning-of-field) (length string)))))
+	(new-end (+ (eww-beginning-of-field) (length string)))
+        (inhibit-read-only t))
     (delete-region start current-end)
     (forward-char offset)
     (insert string
 	    (make-string (- (- (+ new-end offset) start) (length string)) ? ))
-    (if (= 0 offset) (set-text-properties start new-end properties))
+    (when (= 0 offset)
+      (set-text-properties start new-end properties))
     start))
 
 (defun eww-toggle-checkbox ()