ActionMailer content_type bug with 2.3 RC1

I have a bug in my specs when ActionMailer tries to render_message.

wrong number of arguments (0 for 1)
/Library/Ruby/Gems/1.8/gems/actionmailer-2.3.0/lib/action_mailer/base.rb:548:in `content_type'
/Library/Ruby/Gems/1.8/gems/actionmailer-2.3.0/lib/action_mailer/base.rb:548:in `render_message'
/Library/Ruby/Gems/1.8/gems/actionmailer-2.3.0/lib/action_mailer/base.rb:491:in `create!'
/Library/Ruby/Gems/1.8/gems/actionmailer-2.3.0/lib/action_mailer/base.rb:450:in `initialize'
/Library/Ruby/Gems/1.8/gems/actionmailer-2.3.0/lib/action_mailer/base.rb:393:in `new'
/Library/Ruby/Gems/1.8/gems/actionmailer-2.3.0/lib/action_mailer/base.rb:393:in `method_missing'

method_name respond_to :content_type, but method_name.content_type return nothing (not even nil)

@@@ Ruby

  def render_message(method_name, body)
    if method_name.respond_to?(:content_type)
      @current_template_content_type = method_name.content_type  # line 548 here
    end
    render :file => method_name, :body => body
  ensure
    @current_template_content_type = nil
  end

config.action_mailer.delivery_method is set to: :test
When I comment the line 548, this bug doesn't occur anymore but specs are very slow to execute.

I use the edge version of RSpec, but this bug is present too with the last official release of RSpec (1.1.12). I don't think this is related with RSpec.