Source

File: wp-includes/Requests/Response/Headers.php

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

		$key = strtolower($key);

		if (!isset($this->data[$key])) {
			$this->data[$key] = array();
		}

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