Allow client_credentials flow

[szechyjs]

When authenticating using client_credentials flow resource_owner_id is nil. This results in the following exception.

ActiveRecord::RecordNotFound (Couldn't find User without an ID):

activerecord (5.2.1) lib/active_record/relation/finder_methods.rb:429:in `find_with_ids'
activerecord (5.2.1) lib/active_record/relation/finder_methods.rb:69:in `find'
activerecord (5.2.1) lib/active_record/querying.rb:5:in `find'
activerecord (5.2.1) lib/active_record/core.rb:167:in `find'
devise-doorkeeper (1.1.2) lib/devise/strategies/doorkeeper.rb:48:in `resource_from_token'
devise-doorkeeper (1.1.2) lib/devise/strategies/doorkeeper.rb:17:in `authenticate!'
warden (1.2.7) lib/warden/strategies/base.rb:54:in `_run!'
warden (1.2.7) lib/warden/proxy.rb:359:in `block in _run_strategies_for'
warden (1.2.7) lib/warden/proxy.rb:354:in `each'
warden (1.2.7) lib/warden/proxy.rb:354:in `_run_strategies_for'
warden (1.2.7) lib/warden/proxy.rb:324:in `_perform_authentication'
warden (1.2.7) lib/warden/proxy.rb:128:in `authenticate!'
devise (4.5.0) lib/devise/controllers/helpers.rb:118:in `authenticate_user!'

https://github.com/betterup/devise-doorkeeper/blob/master/lib/devise/strategies/doorkeeper.rb#L48

Edit: even if I don't use authenticate_user on a controller ActiveRecord::RecordNotFound (Couldn't find User without an ID) is thrown when using client_credentials. The only way around it is to remove :doorkeeper from the devise config in the User model.

[zousu]

Allow client_credentials flow and optional scopes
#16