Client.php
public function setHeader($key, $value): Client {
		if (null === $key) {
			$this->headers[] = $value;
		}
		else {
			$this->headers[$key] = $value;
		}

		return $this;
	}
public function setHeader($key, $value): Client {
		if (null === $key) {
			$this->headers[] = $value;
		}
		else {
			$this->headers[$key] = $value;
		}

		return $this;
	}
Set header
@param string|null $key
@param string|null $value
@return Client