public function addHeader($header_line)
	{
		$header_parts = explode(':', $header_line, 2);

		if (count($header_parts) !== 2) {
			throw new InvalidArgumentException("'$header_line' is not a valid HTTP header line");
		}

		$header_name  = trim($header_parts[0]);
		$header_value = trim($header_parts[1]);

		if ($this->response->hasHeader($header_name)) {
			$this->response = $this->response->withAddedHeader($header_name, $header_value);
		}
		else {
			$this->response = $this->response->withHeader($header_name, $header_value);
		}

		return $this;
	}