fix parameters for semester index retrieval, fixes #1775

Closes #1775

Merge request studip/studip!1152

lib/classes/JsonApi/Routes/Courses/CoursesIndex.php

@@ -79,7 +79,7 @@ class CoursesIndex extends JsonApiController
             if (!$semester) {
                 return 'Invalid "semester".';
             }
-            $semNumber = \Semester::getIndexById($semester->id);
+            $semNumber = \Semester::getIndexById($semester->id, true, true);
             if ($semNumber === false) {
                 return 'Invalid "semester".';
             }
@@ -101,7 +101,7 @@ class CoursesIndex extends JsonApiController
         $filtering = $this->getQueryParameters()->getFilteringParameters() ?: [];
 
         if (isset($filtering['semester'])) {
-            $filtering['semester'] = \Semester::getIndexById($filtering['semester']);
+            $filtering['semester'] = \Semester::getIndexById($filtering['semester'], true, true);
         }
 
         return array_merge($defaults, $filtering);