update elements and fix some bugs

lib/iron_form.dart

Index: lib/iron_form.dart
diff --git a/lib/iron_form.dart b/lib/iron_form.dart
index 05b1209e09fcee1ae871b47d6da150e7709babdf..387a76792570e80c8797dd5af685082a27eaec8c 100644
--- a/lib/iron_form.dart
+++ b/lib/iron_form.dart
@@ -53,6 +53,13 @@ class IronForm extends FormElement with CustomElementProxyMixin, PolymerBase {
   bool get disableNativeValidationUi => jsElement[r'disableNativeValidationUi'];
   set disableNativeValidationUi(bool value) { jsElement[r'disableNativeValidationUi'] = value; }
 
+  /// HTTP request headers to send
+  ///
+  /// Note: setting a `Content-Type` header here will override the value
+  /// specified by the `contentType` property of this element.
+  get headers => jsElement[r'headers'];
+  set headers(value) { jsElement[r'headers'] = (value is Map || (value is Iterable && value is! JsArray)) ? new JsObject.jsify(value) : value;}
+
   /// Set the withCredentials flag when sending data.
   bool get withCredentials => jsElement[r'withCredentials'];
   set withCredentials(bool value) { jsElement[r'withCredentials'] = value; }