fix(datepicker):selectcorrectmonthandyearfrommonthpickerview…

… when displayMonths > 1

Fixes #4500

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• [ ] added/updated API documentation.
• [ ] added/updated demos.

This question comes from an open source project: valor-software/ngx-bootstrap

Tested in Chrome, FF – latest, looks good. Need to update test for not disabling ts-lint rules and ready for merge.