Allow HalResource without identifier if identifierName is set to false

This change allows registering a single HalResource without an identifier. See for more details this issue:
#109

src/Plugin/HalLinks.php

@@ -546,21 +546,23 @@ public function createResourceFromMetadata($object, Metadata $metadata)
         }
         $data = $hydrator->extract($object);
 
-        $identiferName = $metadata->getIdentifierName();
-        if (!isset($data[$identiferName])) {
+        $identifierName = $metadata->getIdentifierName();
+
+        if (false !== $identifierName && ! isset($data[$identifierName])) {
             throw new Exception\RuntimeException(sprintf(
                 'Unable to determine identifier for object of type "%s"; no fields matching "%s"',
                 get_class($object),
-                $identiferName
+                $identifierName
             ));
         }
-        $id = $data[$identiferName];
+
+        $id = (false === $identifierName) ? null : $data[$identifierName];
 
         $resource = new HalResource($data, $id);
         $links    = $resource->getLinks();
         $this->marshalMetadataLinks($metadata, $links);
         if (!$links->has('self')) {
-            $link = $this->marshalSelfLinkFromMetadata($metadata, $object, $id, $identiferName);
+            $link = $this->marshalSelfLinkFromMetadata($metadata, $object, $id, $identifierName);
             $links->add($link);
         }