Added Gd Encoder Test

tests/Drivers/Gd/Encoders/PngEncoderTest.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace Intervention\Image\Tests\Drivers\Gd\Encoders;
+
+use Intervention\Image\Collection;
+use Intervention\Image\Drivers\Gd\Encoders\PngEncoder;
+use Intervention\Image\Drivers\Gd\Frame;
+use Intervention\Image\Drivers\Gd\Image;
+use Intervention\Image\Tests\TestCase;
+use Intervention\MimeSniffer\MimeSniffer;
+use Intervention\MimeSniffer\Types\ImagePng;
+
+class PngEncoderTest extends TestCase
+{
+    protected function getTestImage(): Image
+    {
+        $frame = new Frame(imagecreatetruecolor(3, 2));
+
+        return new Image(new Collection([$frame]));
+    }
+
+    public function testEncode(): void
+    {
+        $image = $this->getTestImage();
+        $encoder = new PngEncoder();
+        $result = $encoder->encode($image);
+        $this->assertTrue(MimeSniffer::createFromString($result)->matches(new ImagePng));
+    }
+}