fix(grouping): When 'field' in columnDef is referred to some javascript object than a primitive type.
fix(grouping): When 'field' in columnDef is referred to some javascript object than a primitive type.

Grouping fails when 'field' in columnDef is referred to some javascript object
than a primitive type. Resolves issue
[4493](https://github.com/angular-ui/ui-grid/issues/4493)

JayHuang/ng-grid,Servoy/ng-grid,ingshtrom/ui-grid,imbalind/ui-grid,JLLeitschuh/ui-grid,YonatanKra/ui-grid,Cosium/ui-grid,mportuga/ui-grid,Phalynx/ng-grid,kamikasky/ui-grid,angular-ui/ui-grid,JayHuang/ng-grid,dbckr/ui-grid,angular-ui/ng-grid,aw2basc/ui-grid,angular-ui/ng-grid-legacy,kenwilcox/ui-grid,Phalynx/ng-grid,hahn-kev/ui-grid,InteractiveIntelligence/ng-grid,yccteam/ui-grid,angular-ui/ui-grid,dpinho17/ui-grid,InteractiveIntelligence/ng-grid,bjossi86/ui-grid,YonatanKra/ui-grid,Cosium/ui-grid,angular-ui/ui-grid,Servoy/ng-grid,YonatanKra/ui-grid,alex87/ui-grid,JLLeitschuh/ui-grid,imbalind/ui-grid,dlgski/ui-grid,ciccio86/ui-grid,dpinho17/ui-grid,dpinho17/ui-grid,angular-ui/ng-grid,ingshtrom/ui-grid,ciccio86/ui-grid,dlgski/ui-grid,bjossi86/ui-grid,Servoy/ng-grid,angular-ui/ng-grid-legacy,bjossi86/ui-grid,JayHuang/ng-grid,ntmuigrid/ntmuigrid,imbalind/ui-grid,dlgski/ui-grid,ciccio86/ui-grid,mportuga/ui-grid,ntmuigrid/ntmuigrid,aw2basc/ui-grid,AgDude/ng-grid,AgDude/ng-grid,Cosium/ui-grid,JLLeitschuh/ui-grid,hahn-kev/ui-grid,AgDude/ng-grid,kamikasky/ui-grid,alex87/ui-grid,Phalynx/ng-grid,InteractiveIntelligence/ng-grid,aw2basc/ui-grid,mportuga/ui-grid,dbckr/ui-grid,louwie17/ui-grid,louwie17/ui-grid,alex87/ui-grid,kenwilcox/ui-grid,angular-ui/ng-grid-legacy,ntmuigrid/ntmuigrid,angular-ui/ng-grid,hahn-kev/ui-grid,ingshtrom/ui-grid,kenwilcox/ui-grid,louwie17/ui-grid,yccteam/ui-grid,yccteam/ui-grid,dbckr/ui-grid,kamikasky/ui-grid