[BUG] webkit reports incorrect download url

[aslushnikov]

consider the following test:

  it('should report proper download.url() when download is from download attribute', async ({browser, server}) => {
    const page = await browser.newPage({ acceptDownloads: true });
    await page.goto(server.PREFIX + '/empty.html');
    await page.setContent(`<a href="${server.PREFIX}/chromium-linux.zip" download="foo.zip">download</a>`);
    const [ download ] = await Promise.all([
      page.waitForEvent('download'),
      page.click('a')
    ]);
    expect(download.url()).toBe(`${server.PREFIX}/chromium-linux.zip`);
    await page.close();
  });

This test fails on Webkit since download.url() is considered to be page url.