public function setSignature($signature)
    {
        if (!empty($this->signers)) {
            // clear previously set signers
            $swiftMessage = $this->getSwiftMessage();
            foreach ($this->signers as $signer) {
                $swiftMessage->detachSigner($signer);
            }
            $this->signers = [];
        }
        return $this->addSignature($signature);
    }