DateTextBox: make sure that `this.value` is always a `dateClassObj`

Fixes [#18394](https://bugs.dojotoolkit.org/ticket/18394), a problem with hebrew calendars.
Unfortunately I don't have an automated test case since it requires code
in dojox/date to test.

Refs [#5074](https://bugs.dojotoolkit.org/ticket/5074) (the support for specifying non-gregorian calendars), and
refs [#11251](https://bugs.dojotoolkit.org/ticket/11251) (which is when the custom `_set()` method was added that calls `compare()`).