LibWeb: Make XHR.response an actual XMLDocument for XML documents

Before this change, we were producing a generic DOM::Document, which
was not distinguishable from an XMLDocument by IDL interface type.

Tests/LibWeb/Text/expected/XMLHttpRequest-responseXML-gives-XMLDocument.txt

@@ -0,0 +1 @@
+PASS

Tests/LibWeb/Text/input/XMLHttpRequest-responseXML-gives-XMLDocument.html

@@ -0,0 +1,19 @@
+<script src="include.js"></script>
+<script>
+    asyncTest((done) => {
+        const xhr = new XMLHttpRequest();
+        xhr.responseType = "document";
+        xhr.open("GET", "data:text/xml,<?xml version='1.0'?><lol/>", true);
+        xhr.onreadystatechange = function() {
+            if (xhr.readyState === 4 && xhr.status === 200) {
+                let xml = xhr.responseXML;
+                if (xml instanceof XMLDocument)
+                    println("PASS");
+                else
+                    println("FAIL");
+                done();
+            }
+        };
+        xhr.send();
+    });
+</script>

Userland/Libraries/LibWeb/DOM/XMLDocument.cpp

@@ -9,6 +9,11 @@
 
 namespace Web::DOM {
 
+JS::NonnullGCPtr<XMLDocument> XMLDocument::create(JS::Realm& realm, AK::URL const& url)
+{
+    return realm.heap().allocate<XMLDocument>(realm, realm, url);
+}
+
 XMLDocument::XMLDocument(JS::Realm& realm, AK::URL const& url)
     : Document(realm, url)
 {

Userland/Libraries/LibWeb/DOM/XMLDocument.h

@@ -14,6 +14,7 @@ class XMLDocument final : public Document {
     WEB_PLATFORM_OBJECT(XMLDocument, Document);
 
 public:
+    static JS::NonnullGCPtr<XMLDocument> create(JS::Realm&, AK::URL const&);
     virtual ~XMLDocument() override = default;
 
 private:

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2023, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021-2023, Linus Groh <linusg@serenityos.org>
  * Copyright (c) 2022-2023, Luke Wilde <lukew@serenityos.org>
  * Copyright (c) 2022, Ali Mohammad Pur <mpfard@serenityos.org>
@@ -22,6 +22,7 @@
 #include <LibWeb/DOM/Event.h>
 #include <LibWeb/DOM/EventDispatcher.h>
 #include <LibWeb/DOM/IDLEventListener.h>
+#include <LibWeb/DOM/XMLDocument.h>
 #include <LibWeb/Fetch/BodyInit.h>
 #include <LibWeb/Fetch/Fetching/Fetching.h>
 #include <LibWeb/Fetch/Infrastructure/FetchAlgorithms.h>
@@ -317,7 +318,7 @@ void XMLHttpRequest::set_document_response()
 
     // 6. Otherwise, let document be a document that represents the result of running the XML parser with XML scripting support disabled on xhr’s received bytes. If that fails (unsupported character encoding, namespace well-formedness error, etc.), then return null.
     else {
-        document = DOM::Document::create(realm());
+        document = DOM::XMLDocument::create(realm(), m_response->url().value_or({}));
         if (!Web::build_xml_document(*document, m_received_bytes)) {
             m_response_object = Empty {};
             return;