adjust test and cast i18n objects to string, fixes #1164

Closes #1164

Merge request studip/studip!693

lib/classes/JsonApi/Schemas/InstituteMember.php

@@ -21,7 +21,7 @@ class InstituteMember extends SchemaProvider
     public function getAttributes($resource, ContextInterface $context): iterable
     {
         $defaultNull = function ($key) use ($resource) {
-            return $resource->$key ?: null;
+            return (string) $resource->$key ?: null;
         };
 
         $attributes = [

tests/jsonapi/InstitutesMembershipShowTest.php

@@ -23,7 +23,7 @@ class InstitutesMembershipShowTest extends \Codeception\Test\Unit
     public function testShouldShowMemberships()
     {
         $credentials = $this->tester->getCredentialsForTestDozent();
-        $membershipId = '205f3efb7997a0fc9755da2b535038da_2560f7c7674942a7dce8eeb238e15d93';
+        $membershipId = '1';
 
         $app = $this->tester->createApp($credentials, 'get', '/institute-memberships/{id}', InstituteMembershipsShow::class);