Fix Application::run() return values

[weierophinney]

This PR addresses concerns raised in #4831:

• run() now always returns an Application instance; its docblock has been updated accordingly as well.
• If an event returns a response object, it is pushed into the Application instance.

[Maks3w]

Since introduces a BC Break Should this be scheudle for 2.3?

[weierophinney]

@Maks3w We actually changed Application for 2.1 to return the Application instance -- but missed those few returns that occur due to short-circuiting. This just makes it consistent finally.

[reenl]

Docs are still incorrect. http://framework.zend.com/apidoc/2.2/classes/Zend.Mvc.Application.html

[Maks3w]

@reenl This will be released with 2.3

[reenl]

Docs:

$response = $application->run();
$response->send();

2.2.1:
https://github.com/zendframework/zf2/blob/release-2.2.1/library/Zend/Mvc/Application.php#L324
return $this; // Zend\Mvc\Application

In this PR:
https://github.com/weierophinney/zf2/blob/5c70772642323e8d2f2665d5bb921095568e65ee/library/Zend/Mvc/Application.php#L38

 * <code>
 * $services = new Zend\ServiceManager\ServiceManager($servicesConfig);
 * $app      = new Application($appConfig, $services);
 * $app->bootstrap();
 * $response = $app->run();
 * $response->send();
 * </code>

[weierophinney]

@reenl Those docs are generated from a tag. When the new code is released, new docs will be generated.