Add JMS Serializer formatter

To server as example of more flexible formatting.

composer.json

@@ -26,6 +26,7 @@
     "require-dev": {
         "doctrine/coding-standard": "^4.0",
         "infection/infection": "^0.8",
+        "jms/serializer": "^1.11",
         "middlewares/negotiation": "^1.0",
         "phpstan/phpstan": "^0.10@dev",
         "phpstan/phpstan-phpunit": "^0.10@dev",
@@ -35,6 +36,7 @@
         "zendframework/zend-diactoros": "^1.7"
     },
     "suggest": {
+        "jms/serializer": "For content formatting using a more flexible serializer",
         "middlewares/negotiation": "For acceptable format identification",
         "zendframework/zend-diactoros": "For concrete implementation of PSR-7"
     },

src/Formatter/JmsSerializer.php

@@ -0,0 +1,44 @@
+<?php
+declare(strict_types=1);
+
+namespace Lcobucci\ContentNegotiation\Formatter;
+
+use JMS\Serializer\SerializerInterface;
+use Lcobucci\ContentNegotiation\ContentCouldNotBeFormatted;
+use Lcobucci\ContentNegotiation\Formatter;
+use Throwable;
+
+final class JmsSerializer implements Formatter
+{
+    /**
+     * @var SerializerInterface
+     */
+    private $serializer;
+
+    /**
+     * @var string
+     */
+    private $format;
+
+    public function __construct(SerializerInterface $serializer, string $format)
+    {
+        $this->serializer = $serializer;
+        $this->format     = $format;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function format($content): string
+    {
+        try {
+            return $this->serializer->serialize($content, $this->format);
+        } catch (Throwable $exception) {
+            throw new ContentCouldNotBeFormatted(
+                sprintf('Given content could not be formatted in %s using JMS Serializer', $this->format),
+                $exception->getCode(),
+                $exception
+            );
+        }
+    }
+}

tests/Formatter/JmsSerializerTest.php

@@ -0,0 +1,67 @@
+<?php
+declare(strict_types=1);
+
+namespace Lcobucci\ContentNegotiation\Tests\Formatter;
+
+use JMS\Serializer\SerializerInterface;
+use Lcobucci\ContentNegotiation\ContentCouldNotBeFormatted;
+use Lcobucci\ContentNegotiation\Formatter\JmsSerializer;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+/**
+ * @coversDefaultClass \Lcobucci\ContentNegotiation\Formatter\JmsSerializer
+ */
+final class JmsSerializerTest extends TestCase
+{
+    /**
+     * @var SerializerInterface|MockObject
+     */
+    private $serializer;
+
+    /**
+     * @before
+     */
+    public function createSerializer(): void
+    {
+        $this->serializer = $this->createMock(SerializerInterface::class);
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::__construct()
+     * @covers ::format()
+     */
+    public function formatShouldSimplyForwardCallToSerializer(): void
+    {
+        $content = ['a' => 'test'];
+
+        $this->serializer->expects(self::once())
+                         ->method('serialize')
+                         ->with($content, 'json')
+                         ->willReturn('{"a":"test"}');
+
+        $formatter = new JmsSerializer($this->serializer, 'json');
+
+        self::assertSame('{"a":"test"}', $formatter->format($content));
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::__construct()
+     * @covers ::format()
+     */
+    public function formatShouldConvertAnyRaisedException(): void
+    {
+        $this->expectException(ContentCouldNotBeFormatted::class);
+
+        $this->serializer->method('serialize')
+                         ->willThrowException(new RuntimeException());
+
+        $formatter = new JmsSerializer($this->serializer, 'json');
+        $formatter->format(['a' => 'test']);
+    }
+}