clean buffer after exception in route

The output buffer doesn't seem to be cleaned after an error in a route.
This makes the `$this->response->body('');` in the error method to be useless as the route output will be sent to the browser anyway.

````php
$app = new Slim\Slim(array('debug'=>false));
$app->get('/', function() {
    echo 'foo';
    throw new \Exception();
});
$app->error(function() {
    echo 'bar';
});
$app->run();
````
The expected output would be `bar` as the response body is supposed the be cleaned, but the actual output is `foobar` instead.

This PR cleans the buffer before calling the error method.