code: def upload_signing_cert(self, cert_body, user_name=None):
          
          params = {'CertificateBody' : cert_body}
          if user_name:
              params['UserName'] = user_name
          return self.get_response('UploadSigningCertificate', params,
                                   verb='POST')
docs: Uploads an X.509 signing certificate and associates it with
          the specified user.
  
          If the user_name is not specified, it is determined implicitly based
          on the AWS Access Key ID used to sign the request.
  
          :type cert_body: string
          :param cert_body: The body of the signing certificate.
  
          :type user_name: string
          :param user_name: The username of the user