express-serve-static-core-tests: Update req.is to match the actual function in express (#41636)

* Update `req.is` to match the actual function.

The parameters can be a string or an array and return type can be null.
This is described in the JSDoc for the function:
e1b45ebd05/lib/request.js (L273)

* Add a couple of extra tests

types/express-serve-static-core/express-serve-static-core-tests.ts

@@ -9,6 +9,12 @@ app.listen(3000, (err: any) => {
 app.get('/:foo', req => {
     req.params.foo; // $ExpectType string
     req.params[0]; // $ExpectType string
+    // $ExpectType string | false | null
+    req.is(['application/json', 'application/xml']);
+    // $ExpectType string | false | null
+    req.is('audio/wav');
+    // $ExpectError
+    req.is(1);
 });
 
 // Params can used as an array

types/express-serve-static-core/index.d.ts

@@ -7,6 +7,7 @@
 //                 Sami Jaber <https://github.com/samijaber>
 //                 aereal <https://github.com/aereal>
 //                 Jose Luis Leon <https://github.com/JoseLion>
+//                 David Stephens <https://github.com/dwrss>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
@@ -368,7 +369,7 @@ export interface Request<P extends Params = ParamsDictionary, ResBody = any, Req
      *      req.is('html');
      *      // => false
      */
-    is(type: string): string | false;
+    is(type: string | string[]): string | false | null;
 
     /**
      * Return the protocol string "http" or "https"