Allow chaining in Route::setOutputBuffering

[piotr-cz]

After this, it will be possible to chaing setOutputBuffering method just like with setName, setArgument and setArguments methods

$app->get('/foobar', App\Foobar::class)
  ->setOutputBuffering(false)
  ->setName('foobar');

[coveralls]

Coverage remained the same at 97.812% when pulling b7811a8 on piotr-cz:bugfix/chain-setoutputbuffering into 0618c56 on slimphp:3.x.