broken etags / 304 Not modified

Hey, thanks for the great little api.  I just encountered a problem when using Slim's etags... when it is trying to return the 304 Not Modified, slim-json-api picks up the empty response body and sets an error.

I made a quick fix at line 72 of jsonApiMiddleware.php:

```
if ($app->response()->getStatus() != 304 && strlen($app->response()->body()) == 0) {
```
