`Datpicker.value()` validation is too strict

[ctrachte]

setting values when values already exist in Datepicker state:

• start date cannot be set because validation rejects new start date as later than old end date.
  • datepicker.dates state must be reset to an empty array upon receiving 2 new values.

[ctrachte]

Need more sanitization of different date types, and more thorough testing to ensure all values passed in are properly set.