[BUGFIX] existing date values were being cleared

Created by: gracewashere

Fixes #135 (closed)

Problem:

When a user edits a resource with a Date, Time, or DateTime attribute, and the attribute already has a value, the date-time picker clears out the value.

Unless the user re-fills the field, the value will be reset and will cause an error when they hit "submit".

Solution:

Update the datetimepicker date format to match the Rails default.