Unable to pass function name to '_content' property of the route

Follow-up to #1906810: Require type hints for automatic entity upcasting

Problem/Motivation

Commit from #1906810: Require type hints for automatic entity upcasting breaks possibility to pass function name to '_content' property of the route.
For example :

test.route:
  path: 'some/path'
  defaults:
    _title: 'Title'
    _content: 'functionName'

From here : This could be a PHP 4 style function, but best practice in Drupal 8 is to use a controller class.

It is all because of EntityResolverManager::setRouteOptions method. If u look at it you will see that $this->getController
call should return array with controller instance and method name as its values. (docblock comment)

Next. Looking at the getController method.

    if (isset($defaults['_content'])) {
      $controller = $this->controllerResolver->getControllerFromDefinition($defaults['_content']);
    }

And here is it. getControllerFromDefinition method can return just callback function name instead of array.
This scenario is not covered in implementation and u will get en error : "Argument 1 passed to Drupal\Core\Entity\EntityResolverManager::setParametersFromReflection() must be of the type array, string given, called in /home/yarik/www/drupal8/dev/core/lib/Drupal/Core/Entity/EntityResolverManager.php on line 188"

Proposed resolution

Fix it.