AK: Make sure URL retains trailing slash if present in complete_url

Author: https://github.com/deoxxa
Commit: b5b08fba92
Pull-request: https://github.com/SerenityOS/serenity/pull/2261
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/bugaevc

AK/URL.cpp

@@ -323,8 +323,16 @@ URL URL::complete_url(const String& string) const
     auto built = builder.to_string();
     fspath = FileSystemPath(built);
 
+    built = fspath.string();
+    if (string.ends_with('/') && !built.ends_with('/')) {
+        builder.clear();
+        builder.append(built);
+        builder.append('/');
+        built = builder.to_string();
+    }
+
     url = *this;
-    url.set_path(fspath.string());
+    url.set_path(built);
     return url;
 }