src/AbstractTransport.php
public function withHeaders(array $headers)
    {
        $object = clone $this;
        $object->configs['headers'] = [];
        $object->inProcessingLoop = true;
        $object->replaceHeaders($headers);
        $object->inProcessingLoop = true;
        return $object->buildConfigClient();
    }
public function withHeaders(array $headers)
    {
        $object = clone $this;
        $object->configs['headers'] = [];
        $object->inProcessingLoop = true;
        $object->replaceHeaders($headers);
        $object->inProcessingLoop = true;
        return $object->buildConfigClient();
    }
With Set Headers

@param array $headers
@return static