Fix error for value pointing to private items

The code preventing this kind of error was keyed specifically to the
name "resource" for the data type of the value, but we've since
introduced other more specific resource types for values that can point
only to items, sets, etc.

Rather than base this check on the name of the type, this instead
checks if the type class extends from the abstract class for resource
data types. This introduces a new method isHidden to the value
representation to simplify this check.

(fix #1507)

(cherry picked from commit 61abd65)

application/src/Api/Representation/AbstractResourceEntityRepresentation.php

@@ -268,7 +268,7 @@ public function values()
         $values = [];
         foreach ($this->resource->getValues() as $valueEntity) {
             $value = new ValueRepresentation($valueEntity, $this->getServiceLocator());
-            if ('resource' === $value->type() && null === $value->valueResource()) {
+            if ($value->isHidden()) {
                 // Skip this resource value if the resource is not available
                 // (most likely because it is private).
                 continue;

application/src/Api/Representation/ValueRepresentation.php

@@ -1,6 +1,7 @@
 <?php
 namespace Omeka\Api\Representation;
 
+use Omeka\DataType\Resource\AbstractResource;
 use Omeka\Entity\Value;
 use Zend\ServiceManager\ServiceLocatorInterface;
 
@@ -12,7 +13,7 @@ class ValueRepresentation extends AbstractRepresentation
     protected $value;
 
     /**
-     * @var \Omeka\DataType\Manager
+     * @var \Omeka\DataType\DataTypeInterface
      */
     protected $dataType;
 
@@ -168,4 +169,19 @@ public function isPublic()
     {
         return $this->value->isPublic();
     }
+
+    /**
+     * Return whether this value should be hidden when listing values for its
+     * parent resource.
+     *
+     * Currently the only "hidden" values are resource-type values that point
+     * to unretrievable resources (i.e., private resources the current user
+     * cannot see).
+     *
+     * @return bool
+     */
+    public function isHidden()
+    {
+        return $this->dataType instanceof AbstractResource && null === $this->value->getValueResource();
+    }
 }