src/AbstractTransport.php
public function withAddedHeaders(array $headers)
    {
        $object = clone $this;
        $object->inProcessingLoop = false;
        $object->configs['headers'] = [];
        foreach ($headers as $keyName => $value) {
            $keyName = $this->normalizeHeaderName($keyName);
            if (!$keyName) {
                continue;
            }

            if (!isset($this->configs['headers'])) {
                $object->configs['headers'] = [];
            }

            $object->configs['headers'][$keyName] = !is_array($value) ? [$value] : $value;
            $object->request = $object->request->withAddedHeader($keyName, $value);
        }

        return $object->buildConfigClient();
    }
public function withAddedHeaders(array $headers)
    {
        $object = clone $this;
        $object->inProcessingLoop = false;
        $object->configs['headers'] = [];
        foreach ($headers as $keyName => $value) {
            $keyName = $this->normalizeHeaderName($keyName);
            if (!$keyName) {
                continue;
            }

            if (!isset($this->configs['headers'])) {
                $object->configs['headers'] = [];
            }

            $object->configs['headers'][$keyName] = !is_array($value) ? [$value] : $value;
            $object->request = $object->request->withAddedHeader($keyName, $value);
        }

        return $object->buildConfigClient();
    }
With Added Current Headers

@param array $headers
@return static