MarkupInterface objects in config are automatically cast to string during set() and setData()

When running set() or setData() on a configuration object, any objects implementing MarkupInterface within this configuration object will be automatically cast to string. This means that:

$object = new TranslatableMarkup('something bar');
$this->config->set('foo', $object);
// The following will evaluate to TRUE:
$this->config->get('foo') === 'bar';

This has been introduced as a development aid as it should never be necessary to store TranslatableMarkup in configuration.

For more information, see ConfigBase::set(), ConfigBase::setData() and ConfigBase::castSafeStrings().