Update node.d.ts

headers return an object

http://nodejs.org/api/http.html#http_message_headers

node/node.d.ts

@@ -255,7 +255,7 @@ declare module "http" {
     export interface ServerRequest extends events.NodeEventEmitter, stream.ReadableStream {
         method: string;
         url: string;
-        headers: string;
+        headers: any;
         trailers: string;
         httpVersion: string;
         setEncoding(encoding?: string): void;