fix(Headers): serializable toJSON

fixes #6073

Closes #6714

modules/angular2/src/http/headers.ts

@@ -9,6 +9,7 @@ import {
 import {BaseException, WrappedException} from 'angular2/src/facade/exceptions';
 import {
   isListLikeIterable,
+  iterateListLike,
   Map,
   MapWrapper,
   StringMapWrapper,
@@ -131,7 +132,17 @@ export class Headers {
   /**
    * Returns string of all headers.
    */
-  toJSON(): string { return Json.stringify(this.values()); }
+  toJSON(): {[key: string]: any} {
+    let serializableHeaders = {};
+    this._headersMap.forEach((values: string[], name: string) => {
+      let list = [];
+
+      iterateListLike(values, val => list = ListWrapper.concat(list, val.split(',')));
+
+      serializableHeaders[name] = list;
+    });
+    return serializableHeaders;
+  }
 
   /**
    * Returns list of header values for a given name.