When constructing the request URL, ensure that `?` is replaced with `&` when the API root already contains a `?`. Fixes an issue where requests were broken when sites had permalinks set to plain.
Props aduth.
Fixes#42382.
git-svn-id: https://develop.svn.wordpress.org/trunk@42965 602fd350-edb4-49c9-b593-d223f7449a82