Allow multiple passes on the same resource to add missing fields

This solves for a scenario where you have nested includes which result in the
same resource being included twice with different sub-includes. For example, if
`include=author,editor.roles`, and the author and editor point to the same
`users` resource, you still want the `roles` relationship to be loaded on that
user.

Author: tobyzerner

src/Serializer.php

@@ -71,24 +71,28 @@ private function addToMap(ResourceType $resourceType, $model, array $include): a
             $id = $adapter->getId($model)
         );
 
-        if (isset($this->map[$key])) {
-            return $this->map[$key];
+        $url = $resourceType->url($model, $this->context);
+
+        if (!isset($this->map[$key])) {
+            $this->map[$key] = [
+                'type' => $type,
+                'id' => $id,
+                'fields' => [],
+                'links' => [
+                    'self' => new Structure\Link\SelfLink($url),
+                ],
+                'meta' => $this->meta($schema->getMeta(), $model)
+            ];
         }
 
-        $this->map[$key] = [
-            'type' => $type,
-            'id' => $id,
-            'fields' => [],
-            'links' => [
-                'self' => new Structure\Link\SelfLink($url = $resourceType->url($model, $this->context)),
-            ],
-            'meta' => $this->meta($schema->getMeta(), $model)
-        ];
-
         $fields = $this->sparseFields($type, $schema->getFields());
 
         foreach ($fields as $field) {
-            if (! evaluate($field->getVisible(), [$model, $this->context])) {
+            if (isset($this->map[$key]['fields'][$field->getName()])) {
+                continue;
+            }
+
+            if (!evaluate($field->getVisible(), [$model, $this->context])) {
                 continue;
             }