Directions requests made in POST format require access token as part of URL

[danpat]

The Directions and Map Matching APIs support submitting requests via HTTP POST when the size of the request is large.

Per https://docs.mapbox.com/api/navigation/#using-http-post, they need the access_token to be part of the URL string, and all other parameters to be part of the POST body.

It looks like the default behaviour of requests is to put the access_token into the POST body, which will lead to authentication failures if a Directions or Map Matching request is submitted via POST.