Fix parsing values of option elements in forms (#162)

* 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.

* Don't send an <option> tag if it's disabled, even if it's selected

lib/proto/attach-form.js

@@ -67,8 +67,8 @@ 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)
+            if (opt.selected && !opt.disabled) {
+              values.push(opt.hasAttribute("value") ? opt.value : opt.text)
             }
           }
         }