Make Knock more extensible

app/controllers/knock/auth_token_controller.rb

@@ -8,7 +8,7 @@ def create
       render json: auth_token, status: :created
     end
 
-  private
+  protected
     def authenticate
       unless entity.present? && entity.authenticate(auth_params[:password])
         raise Knock.not_found_exception_class

app/model/knock/auth_token.rb

@@ -24,9 +24,13 @@ def entity_for entity_class
         entity_class.find @payload['sub']
       end
     end
+
+    def as_json
+      { jwt: @token }
+    end
 
     def to_json options = {}
-      {jwt: @token}.to_json
+      as_json.to_json
     end
 
   private