Check that a request contains a valid request id.

Addresses [issue #26](https://github.com/GoogleCloudPlatform/appengine-nodejs/issues/26).

The var declaration on this line is making the code less readable.

``` js
if (req.appengine.devappserver) {
    req.appengine.apiTicket = req.appengine.devRequestId;
  }
  if (!goog.isDefAndNotNull(req.appengine.apiTicket)) {
    return new Error('invalid request object: missing request id');
  }
```