`page.data.url` disappears when `page.addData` is called

Description of the bug

When you set processor or preprocessor to call page.addData, page.data.url will be lost.

Reproduction procedure

1. Create index.md at the root directory of the project.
2. Create _config.ts as following:

import lume from "lume/mod.ts";
import { Page, Site } from "lume/core.ts";

const site = lume();

site.use((site: Site) => site.process([".html"], (page: Page) => page.addData({ foo: "bar" })));
// OR: site.use((site: Site) => site.preprocess([".html"], (page: Page) => page.addData({ foo: "bar" })));

site.use((site: Site) => site.addEventListener("afterBuild", () => site.pages.forEach((page) => console.log(page.data))));

export default site;

1. You’ll get the following result:

{ tags: [], content: "index\n", date: 2022-01-10T09:35:35.019Z, foo: "bar" }

Commenting out the processor registration, or passing a function with no side effect as a processor will create the desired output:

{ tags: [], content: "index\n", date: 2022-01-10T09:35:35.019Z, url: "/" }

[oscarotero]

It’s public because it’s used by an external class the Source class but prepending with _ is a good idea.

[aiotter]

Nice work! Thanks 👍