Added ProtocolStreamWriter test

tests/ProtocolStreamWriterTest.php

@@ -0,0 +1,30 @@
+<?php
+declare(strict_types = 1);
+
+namespace LanguageServer\Tests;
+
+use PHPUnit\Framework\TestCase;
+use LanguageServer\ProtocolStreamWriter;
+use LanguageServer\Protocol\Message;
+use AdvancedJsonRpc\{Request as RequestBody};
+
+class ProtocolStreamWriterTest extends TestCase
+{
+    public function testLargeMessageIsSent()
+    {
+        $tmpfile = tempnam('', '');
+        $writeHandle = fopen($tmpfile, 'w');
+
+        stream_set_blocking($writeHandle, false);
+
+        $writer = new ProtocolStreamWriter($writeHandle);
+        $msg = new Message(new RequestBody(1, 'aMethod', ['arg' => str_repeat('X', 100000)]));
+        $msgString = (string)$msg;
+
+        $writer->write($msg);
+
+        fclose($writeHandle);
+
+        $this->assertEquals(strlen($msgString), filesize($tmpfile));
+    }
+}