Fix: Change month without date selected
Fix: Change month without date selected

Fixes https://github.com/alongubkin/angular-datepicker/issues/32

And also replaced `getYear() + 1900` with `getFullYear()`
alongubkin/angular-datepicker,jonathan-reisdorf/angular-datepicker,MrGRA/angular-popup-datepicker,argollo/angular-datepicker,LaminSanneh/angular-datepicker,joohyuntae/testPJT,pacificporter/angular-datepicker