Add formatter that only casts content to string

src/Formatter/StringCast.php

@@ -0,0 +1,24 @@
+<?php
+declare(strict_types=1);
+
+namespace Lcobucci\ContentNegotiation\Formatter;
+
+use Lcobucci\ContentNegotiation\ContentCouldNotBeFormatted;
+use Lcobucci\ContentNegotiation\Formatter;
+use function is_object;
+use function method_exists;
+
+final class StringCast implements Formatter
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function format($content): string
+    {
+        if (is_object($content) && ! method_exists($content, '__toString')) {
+            throw new ContentCouldNotBeFormatted('Given data could not be cast to string');
+        }
+
+        return (string) $content;
+    }
+}

tests/Formatter/StringCastTest.php

@@ -0,0 +1,71 @@
+<?php
+declare(strict_types=1);
+
+namespace Lcobucci\ContentNegotiation\Tests\Formatter;
+
+use Lcobucci\ContentNegotiation\ContentCouldNotBeFormatted;
+use Lcobucci\ContentNegotiation\Formatter\StringCast;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @coversDefaultClass \Lcobucci\ContentNegotiation\Formatter\StringCast
+ */
+final class StringCastTest extends TestCase
+{
+    /**
+     * @test
+     * @dataProvider validData
+     *
+     * @covers ::format()
+     *
+     * @param mixed $content
+     */
+    public function formatShouldSimplyReturnTheStringRepresentationOfTheContent(
+        string $expected,
+        $content
+    ): void {
+        $formatter = new StringCast();
+
+        self::assertSame($expected, $formatter->format($content));
+    }
+
+    /**
+     * @return mixed[][]
+     */
+    public function validData(): array
+    {
+        $test = new class
+        {
+            public function __toString(): string
+            {
+                return 'test';
+            }
+        };
+
+        return [
+            ['test',  'test'],
+            ['test',  $test],
+            ['1',  1],
+            ['1',  true],
+            ['',  false],
+            ['',  null],
+        ];
+    }
+
+    /**
+     * @test
+     *
+     * @covers ::format()
+     */
+    public function formatShouldRaiseExceptionWhenContentCouldNotBeCastToString(): void
+    {
+        $this->expectException(ContentCouldNotBeFormatted::class);
+
+        $content = new class
+        {
+        };
+
+        $formatter = new StringCast();
+        $formatter->format($content);
+    }
+}