Rails 6.1 Compatibility with formats deprecation warning and removal

[tadman]

There's code in mailer_4.rb that trips a deprecation warning in Rails 6.0:

  def each_template(paths, name, &block) #:nodoc:
    (lookup_context.find_all(name, paths) || [ ]).uniq do |t|
      t.formats
    end.each(&block)
  end

Where here formats is no longer supported.