fix(ngModel): do not reset bound date objects

Previously, if you bound a `Date` object to `<input type="time">`,
whenever you changed the time, the day, month, and year fields of
the new resulting bound `Date` object would be reset. Now fields
not modified by bound time input elements are copied to the new
resulting object.

Same for input types of `month`, `week`, etc.

Closes #6666