feat(Headers): implement `toJSON`

modules/angular2/src/http/headers.ts

@@ -1,4 +1,11 @@
-import {isPresent, isBlank, isJsObject, isType, StringWrapper} from 'angular2/src/facade/lang';
+import {
+  isPresent,
+  isBlank,
+  isJsObject,
+  isType,
+  StringWrapper,
+  Json
+} from 'angular2/src/facade/lang';
 import {BaseException, WrappedException} from 'angular2/src/facade/exceptions';
 import {
   isListLikeIterable,
@@ -120,6 +127,11 @@ export class Headers {
    */
   values(): string[][] { return MapWrapper.values(this._headersMap); }
 
+  /**
+   * Returns string of all headers.
+   */
+  toJSON(): string { return Json.stringify(this.values()); }
+
   /**
    * Returns list of header values for a given name.
    */