framework/web/HeaderCollection.php
public function add($name, $value)
    {
        $name = strtolower($name);
        $this->_headers[$name][] = $value;

        return $this;
    }
public function add($name, $value)
    {
        $name = strtolower($name);
        $this->_headers[$name][] = $value;

        return $this;
    }
Adds a new header.
If there is already a header with the same name, the new one will
be appended to it instead of replacing it.
@param string $name the name of the header
@param string $value the value of the header
@return $this the collection object itself