Fix writable serialization of date type with long values

We have fields defined as a date type in the mapping, and we store the millisecond long timestamp for those fields. However, when we are streaming using mapreduce, we get exception saying Date cannot be cast as a Writable.

Initial investigation shows that the `WritableValueReader` class does not map date values coming from its super class `JdkValueReader` to a hadoop writable type.