Avoid to use the config alias while supporting Doorkeeper 5.2

[kymmt90]

The following error has occurred when accessing GET /oauth/userinfo with Doorkeeper 5.2.6 + doorkeeper-openid_connect 1.7.4:

ActionController::RoutingError (undefined method `config' for Doorkeeper:Module
Did you mean?  configure):
F, [2020-11-09T19:36:45.191126 #16] FATAL -- : [866e7cd8-dd2a-4868-b1d9-1dff5caae0bf]   
F, [2020-11-09T19:36:45.191227 #16] FATAL -- : [866e7cd8-dd2a-4868-b1d9-1dff5caae0bf] doorkeeper-openid_connect (1.7.4) app/controllers/doorkeeper/openid_connect/userinfo_controller.rb:6:in `<class:UserinfoController>'
doorkeeper-openid_connect (1.7.4) app/controllers/doorkeeper/openid_connect/userinfo_controller.rb:5:in `<module:OpenidConnect>'
doorkeeper-openid_connect (1.7.4) app/controllers/doorkeeper/openid_connect/userinfo_controller.rb:4:in `<module:Doorkeeper>'
doorkeeper-openid_connect (1.7.4) app/controllers/doorkeeper/openid_connect/userinfo_controller.rb:3:in `<top (required)>'
...

Doorkeeper.configuration has been aliased as Doorkeeper.config from Doorkeeper v5.3.0. This alias has been used in UserInfoController from doorkeeper-openid_connect v1.7.3.

https://github.com/doorkeeper-gem/doorkeeper-openid_connect/blob/2dd6865d011ffacbb07a993e1317395fcb711d4f/app/controllers/doorkeeper/openid_connect/userinfo_controller.rb#L6

However the latest doorkeeper-openid_connect is still supporting Doorkeeper v5.2. and Doorkeeper.config does not work in those versions. To fix this, we should use the original method name while the support of Doorkeeper v5.2. continues.

https://github.com/doorkeeper-gem/doorkeeper-openid_connect/blob/2dd6865d011ffacbb07a993e1317395fcb711d4f/doorkeeper-openid_connect.gemspec#L27

[toupeira]

@kymmt90 thanks for the fix, LGTM! :+1:

Merging...

[kymmt90]

Thanks!