Fix as_json method since application is not required for access_token
Merge pull request #425 from simonbnrd/fix-access_token_issue

Fix as_json method since application is not required for access_token
Update CHANGELOG. README typo fix. [ci skip]
Releasing version 1.3.1. [ci skip]
Resolves to TODOs in the doorkeeper for helper

Moves token validation from DoorkeeperFor class into the Access Token
class.
Add specs for new methods in AccessToken and Scopes

adds test for includes_scope? acceptable?
Fix some of the HoundCI errors
Add Forbidden token response

This resolves isssue #243 by returning 403 Forbidden with a description
of the missing scopes when a token has insufficient scope to request a
resource. Adds a ForbiddenTokenResponse class, and tests for the
implementation.