fix(XhrBackend): setRequestHeader takes a string arg

Closes #4597

modules/angular2/src/http/backends/xhr_backend.ts

@@ -64,7 +64,7 @@ export class XHRConnection implements Connection {
       };
 
       if (isPresent(req.headers)) {
-        req.headers.forEach((value, name) => { _xhr.setRequestHeader(name, value); });
+        req.headers.forEach((values, name) => { _xhr.setRequestHeader(name, values.join(',')); });
       }
 
       _xhr.addEventListener('load', onLoad);

modules/angular2/src/http/headers.ts

@@ -70,7 +70,7 @@ export class Headers {
    */
   delete (name: string): void { MapWrapper.delete(this._headersMap, name); }
 
-  forEach(fn: (value: string, name: string, headers: Headers) => any): void {
+  forEach(fn: (values: string[], name: string, headers: Map<string, string[]>) => void): void {
     MapWrapper.forEach(this._headersMap, fn);
   }