bug#43542: EWW should default checkbox value to "on"

[Nicolas Graner <nicolas.graner@universite-paris-saclay.fr>]

In EWW, when a <input type="checkbox"> has no 'value' attribute, when it
is checked, its value is taken as nil (empty). According to HTML
specification, it should be taken as "on".

Example:

--------------------------------------------------
<form action="foo" method="get">
<input type="checkbox" name="box1" value="bar" checked="checked">
<input type="checkbox" name="box2" checked="checked">
<input type="submit">
</form>
--------------------------------------------------

If you open the above file and hit "submit", the request is:
  foo?box2=&box1=bar
instead of:
  foo?box2=on&box1=bar

Suggested patch:

--------------------------------------------------
diff --git a/lisp/net/eww.el b/lisp/net/eww.el
index 8918be0d2e..55eded6ce6 100644
--- a/lisp/net/eww.el
+++ b/lisp/net/eww.el
@@ -1634,7 +1634,7 @@ eww-submit
          (cond
           ((member (plist-get input :type) '("checkbox" "radio"))
            (when (plist-get input :checked)
-             (push (cons name (plist-get input :value))
+             (push (cons name (or (plist-get input :value) "on"))
                    values)))
           ((equal (plist-get input :type) "file")
             (when-let ((file (plist-get input :filename)))
--------------------------------------------------

Nicolas.