implement node#selected?

lib/capybara/driver/webkit/node.rb

@@ -53,6 +53,10 @@ class Capybara::Driver::Webkit
       invoke("visible") == "true"
     end
 
+    def selected?
+      invoke("selected") == "true"
+    end
+
     def checked?
       self['checked']
     end

src/capybara.js

@@ -91,6 +91,10 @@ Capybara = {
     return true;
   },
 
+  selected: function (index) {
+    return this.nodes[index].selected;
+  },
+
   value: function(index) {
     return this.nodes[index].value;
   },