validate() does not consider 304 a valid response

[Eirik Eikås]

https://github.com/hyperoslo/Malibu/blob/da4354877909d0f1efa86d642f4739290500ceb4/Sources/Response/ResponseValidation.swift#L23

[Vadym Markov]

I don't think 304 response should be considered as success because it doesn't contain a message body. Normally the client should handle 304 status codes and use cached data if it's present.