Can't change skin via API

Trying to create a custom launcher for a modpack so that a player can change his skin inside the launcher.
Using Mojang's API (http://wiki.vg/Mojang_API#Change_Skin), there is an example on how to change the skin via the API.

The problem is: It isn't possible, the server returns a error:

{"error":"Unauthorized","errorMessage":"The request requires user authentication"}

or

{ "error": "Forbidden", "errorMessage": "Current IP not secured"}

Even if I'm passing all the necessary params, including the Header "Authorization: Bearer <token>"

Expected behaviour: Change the skin of the player