Split RequestConcern#on_successful_authorization into before_successful_response and after_successful_response.

This change enables Doorkeeper to be extended by allowing the TokenResponse to be modified following the creation
of an OAuth access token.
Merge pull request #415 from playon/openid_connect_extension

Extend Doorkeeper TokenResponse processing
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]
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.