Fix a bug where the value of <option> would not get sent if falsy

According to the spec, the value attribute should be sent if it
exists, even if it's falsy.

lib/proto/attach-form.js

@@ -68,7 +68,7 @@ function parseFormElements(el) {
           for (var i = 0; i < element.options.length; i++) {
             opt = element.options[i]
             if (opt.selected) {
-              values.push(opt.value || opt.text)
+              values.push(opt.hasAttribute("value") ? opt.value : opt.text)
             }
           }
         }