Application may be nil when using Password Credentials

[fixes #401]
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]
- Adds missing expectations for a spec in protected_resources_controller_spec.rb
Merge pull request #427 from quetzaluz/feature/spec-expectation

- Adds missing expectations for a spec in protected_resources_controller_spec.rb
Don't need to enforce a string-quotes rule
Fix path for specs rubocop rule
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.
Merge pull request #418 from edpaget/handle-forbidden

Requests with insufficient scope should respond 403 Forbidden
Update CHANGELOG. [ci skip]
DRY error response handling
Merge pull request #428 from edpaget/dryer-errors

DRY error response handling
Use RequestConcern's implementation of scopes
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 a style error in specs