Redirect computation fails in some cases on Windows.

What is the current behavior?

npm test results the following error. i've not tracked it down yet but inevitably it will be a path.join problem

 FAIL  tests/routing/deprecated-enterprise-versions.js (71.257 s)
  ● enterprise deprecation › workaround for lost frontmatter redirects works in deprecated enterprise content >=2.13

    expect(received).toBe(expected) // Object.is equality

    Expected: "/en/enterprise/2.15/user/articles/viewing-contributions-on-your-profile"
    Received: "%5Cen%5Centerprise%5C2.15%5Cuser%5Carticles%5Cviewing-contributions-on-your-profile"

      28 |     const res = await get('/en/enterprise/2.15/user/articles/viewing-contributions-on-your-profile-page')
      29 |     expect(res.statusCode).toBe(301)
    > 30 |     expect(res.headers.location).toBe('/en/enterprise/2.15/user/articles/viewing-contributions-on-your-profile')
         |                                  ^
      31 |   })
      32 |
      33 |   test('handles requests for deprecated Enterprise pages ( >=2.13 )', async () => {

      at Object.<anonymous> (tests/routing/deprecated-enterprise-versions.js:30:34)

What changes are you suggesting?

likely should just wrap the join call with a call to slash

cc: @github/docs-engineering , @chiedo