Rename Gateway into PaymentMethod::CreditCard

The `Spree::Gateway` is an implementation of a credit card payment method. The current name is confusing.

With moving this class into the `PaymentMethod` namespace and changing its name to `CreditCard` it is now more obvious what this class actually is.

Several hints inside the class prove that:

- It inherits from `Spree::PaymentMethod`
- The `source_class` is `Spree::CreditCard`
- In `supports?` it asks the gateway if it supports a certain credit card brand

Further actions are required. First on foremost `solidus_gateway` needs to be changed in a way that all credit card based payment methods inherit from `Spree::PaymentMethod::CreditCard` from now on and all non credit card payment methods inherit from `Spree::PaymentMethod` directly.

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## Solidus 2.3.0 (master, unreleased)
 
+- Renamed `Spree::Gateway` payment method into `Spree::PaymentMethod::CreditCard` [\#2001](https://github/com/solidusio/solidus/pull/2001) ([tvdeyen](https://github.com/tvdeyen))
+ Run `rake solidus:migrations:rename_gateways:up` to migrate your data.
+
 - Renamed bogus payment methods [\#2000](https://github.com/solidusio/solidus/pull/2000) ([tvdeyen](https://github.com/tvdeyen))
  `Spree::Gateway::BogusSimple` and `Spree::Gateway::Bogus` were renamed into `Spree::PaymentMethod::SimpleBogusCreditCard` and `Spree::PaymentMethod::BogusCreditCard`
  Run `rake solidus:migrations:rename_gateways:up` to migrate your data.

core/app/models/spree/gateway.rb → core/app/models/spree/payment_method/credit_card.rb

@@ -3,7 +3,7 @@ module Spree
  # base for extension. See https://github.com/solidusio/solidus_gateway/ for
  # offically supported payment gateway implementations.
  #
- class Gateway < PaymentMethod
+ class PaymentMethod::CreditCard < PaymentMethod
  def payment_source_class
  CreditCard
  end

core/lib/solidus/migrations/rename_gateways.rb

@@ -4,6 +4,7 @@ module Solidus
  module Migrations
  class RenameGateways
  DEFAULT_MAPPING = {
+ 'Spree::Gateway' => 'Spree::PaymentMethod::CreditCard',
  'Spree::Gateway::Bogus' => 'Spree::PaymentMethod::BogusCreditCard',
  'Spree::Gateway::BogusSimple' => 'Spree::PaymentMethod::SimpleBogusCreditCard'
  }

core/spec/models/spree/gateway_spec.rb → core/spec/models/spree/payment_method/credit_card_spec.rb

@@ -1,6 +1,6 @@
 require 'spec_helper'
 
-describe Spree::Gateway, type: :model do
+describe Spree::PaymentMethod::CreditCard, type: :model do
  context "fetching payment sources" do
  let(:store) { create :store }
  let(:user) { create :user }