Implemented mainPropertyName method to MailchimpListsSubscription class.

Closes #3315818

modules/mailchimp_lists/src/Plugin/Field/FieldType/MailchimpListsSubscription.php

@@ -89,6 +89,13 @@ class MailchimpListsSubscription extends FieldItemBase {
 
     return $properties;
   }
+  
+  /**
+   * {@inheritdoc}
+   */
+  public static function mainPropertyName() {
+    return 'subscribe';
+  }
 
   /**
    * {@inheritdoc}