kernel/lib/Request/Headers.php
public function set(string $name, string $value = null) : Headers
    {
        if ($value !== null) {
            header($name . ': ' . $value);
        } else {
            header($name);
        }
        
        return $this;
    }
public function set(string $name, string $value = null) : Headers
    {
        if ($value !== null) {
            header($name . ': ' . $value);
        } else {
            header($name);
        }
        
        return $this;
    }
set a new header
@param string $name
@param string $value
@return string|Headers