Routes use _controller instead of _content

Earlier Drupal 8 (including beta 3)

In earlier version of Drupal 8 you had to specify

defaults:
  _content: Drupal\module\Controller\ExampleController::page

in $module.routing.ymlfiles for controllers which return a render array, and _controllerfor things returning a response object.

Drupal 8 (starting from beta 4)

Since #2376791: Move all _content routing definitions to _controller you no longer have to distinguish between the two alternatives, always use _controller

defaults:
  _controller: Drupal\module\Controller\ExampleController::page