when creating an instance with a particular registry, also check for …

…fulfilling instances (fixes #8)

lib/Creation.php

@@ -58,7 +58,7 @@ function create () {
          */
         private function createInstanceWithRegistry (string $className, Creatable $creatable, ResourceRegistry $registry) {
             // Does the registry already contain the resource?
-            $instance = $registry->getClassResource($className);
+            $instance = $registry->getClassResource($className) ?? $registry->findFulfillingInstance($creatable);
             if ($instance !== null) {
                 return $instance;
             }