def set_cookie(name, value, expires)
      (headers['Set-Cookie'] ||=[]) << (Merb::Const::SET_COOKIE % [
        name.to_s,
        ::Merb::Request.escape(value.to_s),
        # Cookie expiration time must be GMT. See RFC 2109
        expires.gmtime.strftime(Merb::Const::COOKIE_EXPIRATION_FORMAT)
      ])
    end