Do not add subject-references at top-level.

jsonld.php

@@ -2770,12 +2770,18 @@ class JsonLdProcessor {
         foreach($subjects_ as $subject_) {
           $node_ = $graph_object->{$subject_};
           unset($node_->listHeadFor);
+          // only add full subjects to top-level
+          if(!self::_isSubjectReference($node_)) {
             $node->{'@graph'}[] = $node_;
           }
         }
+      }
       unset($node->listHeadFor);
+      // only add full subjects to top-level
+      if(!self::_isSubjectReference($node)) {
         $result[] = $node;
       }
+    }
 
     return $result;
   }