String returns the serialization of the cookie for use in a [Cookie]
header (if only Name and Value are set) or a Set-Cookie response
header (if other fields are set).
If c is nil or c.Name is invalid, the empty string is returned.
MaxAge=0 means no 'Max-Age' attribute specified.
MaxAge<0 means delete cookie now, equivalently 'Max-Age: 0'
MaxAge>0 means Max-Age attribute present and given in seconds
A Cookie represents an HTTP cookie as sent in the Set-Cookie header of an HTTP response or the Cookie header of an HTTP request.
See https://tools.ietf.org/html/rfc6265 for details.