refactor(typeahead): change observable registration
refactor(typeahead): change observable registration

Changes in the PR prepare for the fix of #723

Closes #746

ng-bootstrap/ng-bootstrap,benouat/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/ng-bootstrap,ng-bootstrap/core,ACGC/ng-bootstrap,Foxandxss/ng-bootstrap,Xaiid/ng-bootstrap,chenyuzhcy/ng-bootstrap,CrustyJew/ng-bootstrap,maxokorokov/core,ACGC/ng-bootstrap,ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,ktriek/ng-bootstrap,maxokorokov/core,ktriek/ng-bootstrap,benouat/ng-bootstrap,fbasso/ng-bootstrap,deeg/ng-bootstrap,benouat/ng-bootstrap,mschoudry/ng-bootstrap,pkozlowski-opensource/core,deeg/ng-bootstrap,spongessuck/ng-bootstrap,deeg/ng-bootstrap,jnizet/ng-bootstrap,Foxandxss/ng-bootstrap,chenyuzhcy/ng-bootstrap,mschoudry/ng-bootstrap,CrustyJew/ng-bootstrap,maxokorokov/core,ng-bootstrap/ng-bootstrap,benouat/ng-bootstrap,Foxandxss/ng-bootstrap,spongessuck/ng-bootstrap,ktriek/ng-bootstrap,maxokorokov/core,fbasso/ng-bootstrap,mschoudry/ng-bootstrap,ng-bootstrap/core,Xaiid/ng-bootstrap,spongessuck/ng-bootstrap,fbasso/ng-bootstrap,johnbenz13/ng-bootstrap,CrustyJew/ng-bootstrap,jnizet/ng-bootstrap,deeg/ng-bootstrap,Xaiid/ng-bootstrap,ktriek/ng-bootstrap,pkozlowski-opensource/core,ACGC/ng-bootstrap,fbasso/ng-bootstrap,pkozlowski-opensource/core,johnbenz13/ng-bootstrap,CrustyJew/ng-bootstrap,johnbenz13/ng-bootstrap,chenyuzhcy/ng-bootstrap,spongessuck/ng-bootstrap,ng-bootstrap/core