Render DateTime values using ISO 8601 format via XmlRenderer

As reported in #3, the `JsonRenderer` benefits from the fact that PHP
will serialize `DateTime` instances to a JSON object. For the
`XmlRenderer`, we instead detect if the value is a `DateTimeInterface`,
and, if so, call its `format()` method using the "c" format (ISO 8601).

Author: weierophinney

src/Renderer/XmlRenderer.php

@@ -7,6 +7,7 @@
 
 namespace Zend\Expressive\Hal\Renderer;
 
+use DateTimeInterface;
 use DOMDocument;
 use DOMNode;
 use Zend\Expressive\Hal\HalResource;
@@ -152,17 +153,20 @@ private function createNodeTree(DOMDocument $doc, DOMNode $node, array $data) :
      * @todo Detect JsonSerializable, and pass to
      *     json_decode(json_encode($object), true), passing the final value
      *     back to createResourceElement()?
-     * @todo How should we handle DateTimeInterface implementations?
-     *     $date->format('c')?
      * @param object $object
      * @throws Exception\InvalidResourceValueException if unable to serialize
      *     the data to a string.
      */
     private function createDataFromObject($object) : string
     {
+        if ($object instanceof DateTimeInterface) {
+            return $object->format('c');
+        }
+
         if (! method_exists($object, '__toString')) {
             throw Exception\InvalidResourceValueException::fromObject($object);
         }
+
         return (string) $object;
     }
 }

test/Renderer/XmlRendererTest.php

@@ -65,6 +65,9 @@ public function testRendersExpectedXmlPayload()
         $this->assertSame($expected, $renderer->render($resource));
     }
 
+    /**
+     * @see https://github.com/zendframework/zend-expressive-hal/issues/3
+     */
     public function testCanRenderPhpDateTimeInstances()
     {
         $dateTime = new DateTime('now');
@@ -75,6 +78,6 @@ public function testCanRenderPhpDateTimeInstances()
 
         $renderer = new XmlRenderer();
         $xml = $renderer->render($resource);
-        $this->assertContains((string) $dateTime, $xml);
+        $this->assertContains($dateTime->format('c'), $xml);
     }
 }