Ensure run() always returns Application instance

- Some paths through `Application::run()` would call
  `return $this->completeRequest($event)`; this particular method was returning
  a response object, which meant that `send()` was resulting in a double-render.
  The main `run()` routine typically returns the Application instance, where
  `send()` is a no-op, and thus no double-render can occur.
- The workaround until this is merged and released is to remove the call to
  `->send()` in your `public/index.php`.
Merge pull request #3648 branch 'hotfix/fix-double-render'