Implement failing test case for #3, and initial solution

Adds a failing test case demonstrating that `DateTime` values trigger an
exception currently. I also propose a solution that currently addresses
objects with a `__toString()` mechanism. This does not solve the case of
`DateTime`, however, as it does not implement that method.

Author: weierophinney

src/Exception/InvalidResourceValueException.php

@@ -20,4 +20,16 @@ public static function fromValue($value) : self
             HalResource::class
         ));
     }
+
+    /**
+     * @param object $object
+     */
+    public static function fromObject($object) : self
+    {
+        return new self(sprintf(
+            'Encountered object of type "%s" when serializing %s instance; unable to serialize',
+            get_class($object),
+            HalResource::class
+        ));
+    }
 }

src/Renderer/XmlRenderer.php

@@ -112,6 +112,11 @@ private function createResourceElement(DOMDocument $doc, string $name, $data)
             return $doc->createElement($name, $data);
         }
 
+        if (is_object($data)) {
+            $data = $this->createDataFromObject($data);
+            return $doc->createElement($name, $data);
+        }
+
         if (! is_array($data)) {
             throw Exception\InvalidResourceValueException::fromValue($data);
         }
@@ -142,4 +147,22 @@ private function createNodeTree(DOMDocument $doc, DOMNode $node, array $data) :
 
         return $node;
     }
+
+    /**
+     * @todo Detect JsonSerializable, and pass to
+     *     json_decode(json_encode($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 (! method_exists($object, '__toString')) {
+            throw Exception\InvalidResourceValueException::fromObject($object);
+        }
+        return (string) $object;
+    }
 }

test/Renderer/XmlRendererTest.php

@@ -7,7 +7,10 @@
 
 namespace ZendTest\Expressive\Hal\Renderer;
 
+use DateTime;
 use PHPUnit\Framework\TestCase;
+use Zend\Expressive\Hal\HalResource;
+use Zend\Expressive\Hal\Link;
 use Zend\Expressive\Hal\Renderer\XmlRenderer;
 
 class XmlRendererTest extends TestCase
@@ -61,4 +64,17 @@ public function testRendersExpectedXmlPayload()
 
         $this->assertSame($expected, $renderer->render($resource));
     }
+
+    public function testCanRenderPhpDateTimeInstances()
+    {
+        $dateTime = new DateTime('now');
+        $resource = new HalResource([
+            'date' => $dateTime,
+        ]);
+        $resource = $resource->withLink(new Link('self', '/example'));
+
+        $renderer = new XmlRenderer();
+        $xml = $renderer->render($resource);
+        $this->assertContains((string) $dateTime, $xml);
+    }
 }