Update Polymer components.

third_party/polymer/components/iron-form/iron-form.html

 <!--
 @license
 Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
 This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
 The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
 The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
 Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 -->
 
@@ skipping 216 matching lines @@
         this.fire('iron-form-invalid');
         return;
       }
 
       var json = this.serialize();
 
       // Native forms can also index elements magically by their name (can't make
       // this up if I tried) so we need to get the correct attributes, not the
       // elements with those names.
       this.request.url = this.getAttribute('action');
-      this.request.method = this.getAttribute('method');
+      this.request.method = this.getAttribute('method') || 'GET';
       this.request.contentType = this.contentType;
       this.request.withCredentials = this.withCredentials;
       this.request.headers = this.headers;
 
       if (this.request.method.toUpperCase() === 'POST') {
         this.request.body = json;
       } else {
         this.request.params = json;
       }
 
@@ skipping 37 matching lines @@
      * array of values.
      *
      * @return {!Object}
      */
     serialize: function() {
       var json = {};
 
       function addSerializedElement(name, value) {
         // If the name doesn't exist, add it. Otherwise, serialize it to
         // an array,
-        if (!json[name]) {
+        if (json[name] === undefined) {
           json[name] = value;
         } else {
           if (!Array.isArray(json[name])) {
             json[name] = [json[name]];
           }
           json[name].push(value);
         }
       }
 
       // Go through all of the registered custom components.
@@ skipping 198 matching lines @@
             parent._parentForm === this) {
           return true;
         }
       }
       return false;
     }
 
   });
 
 </script>