src/Message.php
protected function setHeaders(array $headers): MessageInterface
    {
        foreach ($headers as $name => $value) {
            $name = mb_convert_case($name, MB_CASE_TITLE);
            $this->headers[$name] = (array) $value;
        }

        return $this;
    }
protected function setHeaders(array $headers): MessageInterface
    {
        foreach ($headers as $name => $value) {
            $name = mb_convert_case($name, MB_CASE_TITLE);
            $this->headers[$name] = (array) $value;
        }

        return $this;
    }
Set headers.

@param string[] $headers Headers.

@return static