def list(domain=nil)
      if domain
        @mailgun.response = Mailgun::Base.fire(:get, @mailgun.api_url + "/domains/#{domain}")
      else
        @mailgun.response = Mailgun::Base.fire(:get, @mailgun.api_url + "/domains")["items"] || []
      end
    end
def list(domain=nil)
      if domain
        @mailgun.response = Mailgun::Base.fire(:get, @mailgun.api_url + "/domains/#{domain}")
      else
        @mailgun.response = Mailgun::Base.fire(:get, @mailgun.api_url + "/domains")["items"] || []
      end
    end
Used internally
 List Domains. If domain name is passed return detailed information, otherwise return a list of all domains.