DEV: Fix policy classes delegating their #call method in services

There’s currently a bug when using a dedicated class as a policy in
services: if that class delegates its `#call` method (to an underlying
strategy object for example), then an error will be raised saying steps
aren’t allowed to provide default parameters.

This should not happen, and this patch fixes that issue.

spec/lib/service_spec.rb

@@ -38,6 +38,36 @@ RSpec.describe Service do
           expect { service_class.call }.to raise_error(/In policy 'my_policy': default values/)
         end
       end
+
+      context "when providing a class which delegates its `#call` method" do
+        before do
+          service_class.class_eval do
+            class MyPolicy < Service::PolicyBase
+              class MyStrategy
+                def call
+                end
+
+                def reason
+                end
+              end
+
+              attr_reader :strategy
+
+              delegate :call, :reason, to: :strategy
+
+              def initialize(*)
+                @strategy = MyStrategy.new
+              end
+            end
+
+            policy :my_policy, class_name: MyPolicy
+          end
+        end
+
+        it "does not raise an error" do
+          expect { service_class.call }.not_to raise_error
+        end
+      end
     end
 
     describe "Generic step" do