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);
   }
 

modules/angular2/test/http/backends/xhr_backend_spec.ts

@@ -148,14 +148,16 @@ export function main() {
       });
 
       it('should attach headers to the request', () => {
-        var headers = new Headers({'Content-Type': 'text/xml', 'Breaking-Bad': '<3'});
+        var headers =
+            new Headers({'Content-Type': 'text/xml', 'Breaking-Bad': '<3', 'X-Multi': ['a', 'b']});
 
         var base = new BaseRequestOptions();
         var connection = new XHRConnection(
             new Request(base.merge(new RequestOptions({headers: headers}))), new MockBrowserXHR());
         connection.response.subscribe();
-        expect(setRequestHeaderSpy).toHaveBeenCalledWith('Content-Type', ['text/xml']);
-        expect(setRequestHeaderSpy).toHaveBeenCalledWith('Breaking-Bad', ['<3']);
+        expect(setRequestHeaderSpy).toHaveBeenCalledWith('Content-Type', 'text/xml');
+        expect(setRequestHeaderSpy).toHaveBeenCalledWith('Breaking-Bad', '<3');
+        expect(setRequestHeaderSpy).toHaveBeenCalledWith('X-Multi', 'a,b');
       });
 
       it('should return the correct status code', inject([AsyncTestCompleter], async => {