fix(js): default checkbox and radio value to "on"

A checkbox or radio input with no value attribute returns "on" in a real
browser; obscura returned the empty string. Page scripts that read
input.value to build form submissions (and browser-environment checks)
saw the wrong default. An explicit value attribute still wins.

Refs #335.

crates/obscura-js/js/bootstrap.js

@@ -1469,6 +1469,15 @@ class Element extends Node {
       const attr = this.getAttribute('value');
       return attr !== null ? attr : this.textContent;
     }
+    if (tag === 'input') {
+      const itype = (this.getAttribute('type') || '').toLowerCase();
+      if (itype === 'checkbox' || itype === 'radio') {
+        // A checkbox/radio with no value attribute defaults to "on" in a real
+        // browser, not the empty string.
+        const attr = this.getAttribute('value');
+        return attr !== null ? attr : 'on';
+      }
+    }
     return this.getAttribute("value") || "";
   }
   set value(v) {