public function offsetSet($key, $value) {
		if (is_null($key)) {
			throw new Requests_Exception('Headers is a dictionary, not a list', 'invalidset');
		}

		$key = strtolower($key);

		if (isset($this->data[$key])) {
			// RFC2616 notes that multiple headers must be able to
			// be combined like this. We should use a smarter way though (arrays
			// internally, e.g.)
			$value = $this->data[$key] . ',' . $value;
		}

		$this->data[$key] = $value;
	}