public function withHeader(string $key, string $value): void
    {
        $this->headers[$key] = $value;
    }
public function withHeader(string $key, string $value): void
    {
        $this->headers[$key] = $value;
    }
public function withHeader ( string $ key , string $ value ) : void { $ this -> headers [ $ key ] = $ value ; }
Write header value.

@param string $key
@param string $value
@return void
Write header value .