LibWeb: Implement XMLHttpRequest.responseURL

This was used on https://twinings.co.uk/ so let's support it :^)

Tests/LibWeb/Text/expected/XHR/XMLHttpRequest-responseURL.txt

@@ -0,0 +1,2 @@
+responseURL before: ''
+responseURL after: 'data:text/html,hello'

Tests/LibWeb/Text/input/XHR/XMLHttpRequest-responseURL.html

@@ -0,0 +1,15 @@
+<script src="../include.js"></script>
+<script>
+    asyncTest((done) => {
+        const xhr = new XMLHttpRequest();
+        xhr.open("GET", "data:text/html,hello", true);
+        println("responseURL before: '" + xhr.responseURL + "'");
+        xhr.onreadystatechange = function() {
+            if (xhr.readyState === 4 && xhr.status === 200) {
+                println("responseURL after: '" + xhr.responseURL + "'");
+                done();
+            }
+        };
+        xhr.send();
+    });
+</script>

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -1265,4 +1265,16 @@ JS::ThrowCompletionOr<void> XMLHttpRequest::request_error_steps(FlyString const&
     return {};
 }
 
+// https://xhr.spec.whatwg.org/#the-responseurl-attribute
+String XMLHttpRequest::response_url()
+{
+    // The responseURL getter steps are to return the empty string if this’s response’s URL is null;
+    // otherwise its serialization with the exclude fragment flag set.
+    if (!m_response->url().has_value())
+        return String {};
+
+    auto serialized = m_response->url().value().serialize(URL::ExcludeFragment::Yes);
+    return String::from_utf8_without_validation(serialized.bytes());
+}
+
 }

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.h

@@ -52,6 +52,7 @@ public:
     WebIDL::ExceptionOr<JS::GCPtr<DOM::Document>> response_xml();
     WebIDL::ExceptionOr<JS::Value> response();
     Bindings::XMLHttpRequestResponseType response_type() const { return m_response_type; }
+    String response_url();
 
     WebIDL::ExceptionOr<void> open(String const& method, String const& url);
     WebIDL::ExceptionOr<void> open(String const& method, String const& url, bool async, Optional<String> const& username = Optional<String> {}, Optional<String> const& password = Optional<String> {});

Userland/Libraries/LibWeb/XHR/XMLHttpRequest.idl

@@ -41,7 +41,7 @@ interface XMLHttpRequest : XMLHttpRequestEventTarget {
     undefined abort();
 
     // response
-    // FIXME: readonly attribute USVString responseURL;
+    readonly attribute USVString responseURL;
     readonly attribute unsigned short status;
     readonly attribute ByteString statusText;
     ByteString? getResponseHeader(ByteString name);