wrong url in authorize

There is an error in the authorize method which is making the method fail with a 404 - Not Found http error. The problems appears to be that, in line 134 of Mastodon::Client the url passed to post is 'oauth/token'. This makes the user agent trying to post on https://mastodon.example/api/v1/oauth/token (which doesn't exist) instead of the correct https://mastodon.example/oauth/token. It is enough to add a slash at the beginning, as in '/oauth/token' making the path absolute to solve the problem. I may provide a patch, if needed.