Check which methods we need to delegate

This implicit delegation makes it impossible to know which code is used
and which code is dead. The refund method is very rarely used though. So
we'll need to wait for a while.

app/models/spree/gateway.rb

@@ -5,7 +5,7 @@ module Spree
     acts_as_taggable
     include PaymentMethodDistributors
 
-    delegate :authorize, :purchase, :capture, :void, :credit, to: :provider
+    delegate :authorize, :purchase, :capture, :void, :credit, :refund, to: :provider
 
     validates :name, :type, presence: true
 
@@ -35,6 +35,10 @@ module Spree
     end
 
     def method_missing(method, *)
+      message = "Deprecated delegation of Gateway##{method}"
+      Alert.raise(message)
+      raise message if Rails.env.local?
+
       if @provider.nil? || !@provider.respond_to?(method)
         super
       else

spec/models/spree/gateway_spec.rb

@@ -14,8 +14,14 @@ RSpec.describe Spree::Gateway do
   end
 
   it "passes through all arguments on a method_missing call" do
+    expect(Rails.env).to receive(:local?).and_return(false)
     gateway = test_gateway.new
     expect(gateway.provider).to receive(:imaginary_method).with('foo')
     gateway.imaginary_method('foo')
   end
+
+  it "raises an error in test env" do
+    gateway = test_gateway.new
+    expect { gateway.imaginary_method('foo') }.to raise_error StandardError
+  end
 end