fix(http): Headers.append should append to the list

modules/@angular/http/src/headers.ts

@@ -88,7 +88,12 @@ export class Headers {
    */
   append(name: string, value: string): void {
     const values = this.getAll(name);
-    this.set(name, values === null ? [value] : [...values, value]);
+
+    if (values === null) {
+      this.set(name, value);
+    } else {
+      values.push(value);
+    }
   }
 
   /**

modules/@angular/http/test/headers_spec.ts

@@ -111,6 +111,14 @@ export function main() {
     });
 
     describe('.append', () => {
+      it('should append a value to the list', () => {
+        const headers = new Headers();
+        headers.append('foo', 'bar');
+        headers.append('foo', 'baz');
+        expect(headers.get('foo')).toEqual('bar');
+        expect(headers.getAll('foo')).toEqual(['bar', 'baz']);
+      });
+
       it('should preserve the case of the first call', () => {
         const headers = new Headers();