Add Document encoding to header preloaded requests

third_party/WebKit/LayoutTests/http/tests/preload/link_preload_charset.php

+<?php
+header("Content-Type: text/html;charset=utf-8", false);
+header("Link: <http://127.0.0.1:8000/preload/resources/charset.js>;rel=preload;as=script", false);
+?>
+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<script>
+    var t = async_test("Makes sure that header preloaded resources are downloaded with the document's charset");
+    window.addEventListener("load", t.step_func(function() {
+        assert_equals(window.preload_header, 'Кириллиця');
+        t.done();
+    }));
+</script>
+<script src="resources/charset.js"></script>