ENYO-2799: Disabled serialization of "value" property for onyx.DatePicker and onyx.TimePicker

Enyo-DCO-1.1-Signed-off-by: Yves Del Medico <yves.del-medico@hp.com>
Merge pull request #169 from enyojs/ENYO-2799

ENYO-2799: Disabled serialization of "value" property for onyx.DatePicke...

Reviewed-By: Ben Combee (ben.combee@lge.com)