Reject requests without mandatory custom fields

We need to make sure subscribers cannot be created without custom fields
Users require GDPR consent and we need to make sure there are no way to
create a subscriber without mandatory custom fields

[MAILPOET-1405]

tests/unit/API/MP/APITest.php

@@ -301,6 +301,21 @@ class APITest extends \MailPoetTest {
     expect($result['source'])->equals('api');
   }
 
+  function testItChecksForMandatoryCustomFields() {
+    CustomField::createOrUpdate([
+      'name' => 'custom field',
+      'type' => 'text',
+      'params' => ['required' => '1']
+    ]);
+
+    $subscriber = array(
+      'email' => 'test@example.com',
+    );
+
+    $this->setExpectedException('Exception');
+    API::MP(self::VERSION)->addSubscriber($subscriber);
+  }
+
   function testItSubscribesToSegmentsWhenAddingSubscriber() {
     $API = Stub::makeEmptyExcept(
       new \MailPoet\API\MP\v1\API(),