Fix exception when mask isn't set or is false.
Fix exception when mask isn't set or is false.

Added an additional check to ensure options.mask is a string as it is set to false when not using masks.
Fixes #414
nitishdola/material-management-systems,nitishdola/material-management-systems,andrewmnlv/datetimepicker,xdan/datetimepicker,andrewmnlv/datetimepicker,nitishdola/material-management-systems,nitishdola/material-management-systems,xdan/datetimepicker