DateTime serialization

[Rui Marinho]

When serializing a DateTime object without the single_text form option, the serialized response is empty:

"{"name":"sample","created_at":{"date":{"year":"","month":"","day":""},"time":{"hour":"","minute":""}}}"

Same object with single_text:

"{"name":"sample","created_at":"2012-07-20T15:49:59Z"}"