Fixes for 2 digit years

Datepicker: fixed problem with years between 1 and 99 (but broke two tests and I cannot figure why, see below). Fixed #7098 - Datepicker: Broken handling of four digit years before year 100


Broke test 21:18
```
Year Range Test - previous button disabled at 1/1/minYear
Source: 	
@file:///git/jquery-ui/tests/unit/datepicker/datepicker_options.js:493
```

Broke test 24:15
```
Auto size - he - DD, MM dd, yy
Expected: 	

23

Result: 	

21

Diff: 	

23 21 

Source: 	

@file:///git/jquery-ui/tests/unit/datepicker/datepicker_options.js:668
```