small tweaks using `text_part` and slightly changed regex

devise_mail_helpers.rb

module Features
  module MailHelpers

    def last_email
      ActionMailer::Base.deliveries.last
    end

    # Can be used like:
    #  extract_token_from_email(:reset_password)
    def extract_token_from_email(token_name)
      mail_body = last_email.text_part.to_s
      mail_body[/#{token_name.to_s}_token=([^"\s]+)/, 1]
    end
  end
end