PushQueue causes fatal error in Drupal\Core\Database\Query\Merge

Problem/Motivation

Drupal\Core\Database\Query\Merge::key() is called with an array of keys. That method in Core throws a fatal error if its not a string:

public function key($field, $value = NULL) {
  assert(is_string($field));
  $this->keys([$field => $value]);
  return $this;
}

Steps to reproduce

Check PushQueue code

Proposed resolution

Call Drupal\Core\Database\Query\Merge::keys() instead which accepts array of keys

Remaining tasks

MR

User interface changes

N/A

API changes

N/A

Data model changes

N/A