Fix master linking for localized posts

[villesiltala]

The documentation describes master linking with the following specifications:

• i18n (object) (Optional) Custom localization information is stored in the property. It must contain an object with the following properties:
  • locale (string) (Required) The language code string, for example fi.
  • master (string|object) (Required) A gi_id value to be used to fetch a default language post or an array containing a query_key value. This is used to link the post as a translation.

It seems that the current version only supports the array type as the "master" property. The validation does not allow passing a value other than an array with a "query_key" key. See:
https://github.com/devgeniem/wp-geniem-importer/blob/master/src/Post.php#L593

Fix this by allowing a direct id value to be set as the value of "master".