Argument for Merge::key() should not be an array

Problem/Motivation

We are getting this assertion error when saving a node with an overridden locale:

AssertionError:
assert(is_string($field))

  at core/lib/Drupal/Core/Database/Query/Merge.php:330
  at assert(false, 'assert(is_string($field))')
     (core/lib/Drupal/Core/Database/Query/Merge.php:330)
  at Drupal\Core\Database\Query\Merge->key(array('nid' => '388'))
     (modules/contrib/html_lang_override/src/HtmlLangManager.php:137)
  at Drupal\html_lang_override\HtmlLangManager->saveLangForNode('388', 'gezg')
     (modules/contrib/html_lang_override/html_lang_override.module:59)
  at html_lang_override_entity_presave(object(Node))

Steps to reproduce

Edit a node and fill the "Custom HTML Lang Attribute" field.

Proposed resolution

I think key() should be called like this:

->key('nid', $nid)

Remaining tasks

User interface changes

API changes

Data model changes