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

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

        return $this;
    }
Sets a new header only if it does not exist yet.
If there is already a header with the same name, the new one will be ignored.
@param string $name the name of the header
@param string $value the value of the header
@return $this the collection object itself