Encode mail as quoted-printable before sending in Rails

I saw this question and couldn't make use of it: Ruby email encoding and quoted-printable content

I'd like to send out开发者_如何转开发 email as quoted-printable, but it's going out as unencoded text/html. Any help?

class UserMailer < ActionMailer::Base

  def welcome_mail(user)
    recipients        user.email
    from              "no-reply@example.com"
    subject           "Thanks for registering"
    body              :user => user
  end

end

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There is a solution posted on Ruby Quiz.