SPARK-4228 SchemaRDD to JSON

Here's a simple fix for SchemaRDD to JSON.
If we use string for a timestamp value, the meaning of the time can be changed (e.g. the data is generated by a developer in a time zone and then it is read by another developer in another time zone). I feel using `getTime` is better (it is not very reader friendly though).

@marmbrus What do you think?