use php 7.2 for tests and fix error in jsonapi, fixes #2166

Closes #2166

Merge request studip/studip!1400

.gitlab-ci.yml

-image: studip/studip:tests
+image: studip/studip:tests-php7.2
 
 variables:
   MYSQL_RANDOM_ROOT_PASSWORD: "true"

lib/classes/JsonApi/Routes/Files/FileRefsCreate.php

@@ -14,7 +14,10 @@ use JsonApi\Schemas\File as FileSchema;
 
 class FileRefsCreate extends JsonApiController
 {
-    use RangeHelperTrait, RoutesHelperTrait, ValidationTrait;
+    use RangeHelperTrait, RoutesHelperTrait, ValidationTrait {
+        ValidationTrait::arrayGet insteadof RoutesHelperTrait;
+        ValidationTrait::arrayHas insteadof RoutesHelperTrait;
+    }
 
     /**
      * @SuppressWarnings(PHPMD.UnusedFormalParameters)