fix(datepicker): add class "show" to visible datepicker dropdown
fix(datepicker): add class "show" to visible datepicker dropdown

Fixes #2088
Closes #2013

fbasso/ng-bootstrap,jnizet/ng-bootstrap,jnizet/ng-bootstrap,benouat/ng-bootstrap,ng-bootstrap/ng-bootstrap,ktriek/ng-bootstrap,benouat/ng-bootstrap,benouat/ng-bootstrap,ktriek/ng-bootstrap,ktriek/ng-bootstrap,maxokorokov/core,jnizet/ng-bootstrap,ktriek/ng-bootstrap,fbasso/ng-bootstrap,fbasso/ng-bootstrap,maxokorokov/core,maxokorokov/core,maxokorokov/core,ng-bootstrap/ng-bootstrap,benouat/ng-bootstrap,ng-bootstrap/ng-bootstrap,ng-bootstrap/core,pkozlowski-opensource/core,pkozlowski-opensource/core,ng-bootstrap/core,ng-bootstrap/core,jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,fbasso/ng-bootstrap,pkozlowski-opensource/core