Move deliver_later gem logic into HaveEnqueuedMail matcher

Moving the internals of
https://github.com/jdlubrano/deliver_later_matchers into rspec-rails as
a new RSpec::Rails::Matchers::HaveEnqueuedMail class.

Author: Joel Lubrano

lib/rspec/rails/matchers/have_enqueued_mail.rb

@@ -0,0 +1,87 @@
+require "rspec/rails/matchers/active_job"
+
+module RSpec
+  module Rails
+    module Matchers
+      # @api private
+      #
+      # Matcher class for `have_enqueued_mail`. Should not be instantiated directly.
+      #
+      # @see RSpec::Rails::Matchers#have_enqueued_mail
+      class HaveEnqueuedMail < RSpec::Matchers::BuiltIn::BaseMatcher
+        def initialize(mailer_class, method_name)
+          @mailer_class = mailer_class
+          @method_name = method_name
+          @args = []
+        end
+
+        def description
+          "enqueues #{@mailer_class.name}.#{@method_name}"
+        end
+
+        def matches?(block)
+          raise ArgumentError, 'have_enqueued_mail and enqueue_mail only work with block arguments' unless block.respond_to?(:call)
+          check_active_job_adapter
+
+          job_matcher = ActiveJob::HaveEnqueuedJob.new(ActionMailer::DeliveryJob)
+          job_matcher.with(*mailer_args)
+          job_matcher.matches?(block)
+        end
+
+        def with(*args)
+          @args = args
+          self
+        end
+
+        def failure_message
+          base_message.tap do |msg|
+            msg << " with #{@args}" if @args.any?
+          end
+        end
+
+        def supports_block_expectations?
+          true
+        end
+
+        private
+
+        def base_message
+          "expected to enqueue #{@mailer_class.name}.#{@method_name}"
+        end
+
+        def mailer_args
+          [@mailer_class.name, @method_name.to_s, 'deliver_now'] + @args
+        end
+
+        def check_active_job_adapter
+          return if ::ActiveJob::QueueAdapters::TestAdapter === ::ActiveJob::Base.queue_adapter
+          raise StandardError, "To use HaveEnqueuedMail matcher set `ActiveJob::Base.queue_adapter = :test`"
+        end
+      end
+
+      # @api public
+      # Passes if an email has been enqueued inside block. May chain with to specify expected arguments.
+      #
+      # @example
+      #     expect {
+      #       MyMailer.welcome(user).deliver_later
+      #     }.to have_enqueued_mail(MyMailer, :welcome)
+      #
+      #     # Using alias
+      #     expect {
+      #       MyMailer.welcome(user).deliver_later
+      #     }.to enqueue_mail(MyMailer, :welcome)
+      #
+      #     expect {
+      #       MyMailer.welcome(user).deliver_later
+      #     }.to have_enqueued_mail(MyMailer, :welcome).with(user)
+      def have_enqueued_mail(mailer_class, mail_method_name)
+        check_active_job_adapter
+        HaveEnqueuedMail.new(mailer_class, mail_method_name)
+      end
+      alias_method :have_enqueued_email, :have_enqueued_mail
+      alias_method :enqueue_mail, :have_enqueued_mail
+      alias_method :enqueue_email, :have_enqueued_mail
+    end
+  end
+end