fix(group-by): use header instead of field in the default groupby row #6939

Author: dkamburov

projects/igniteui-angular/src/lib/grids/grid/groupby-row.component.html

@@ -22,7 +22,9 @@
         <div class="igx-group-label">
             <igx-icon fontSet="material" class="igx-group-label__icon">group_work</igx-icon>
             <span class="igx-group-label__column-name">
-            {{ groupRow.expression ? groupRow.expression.fieldName : '' }}:
+            {{ groupRow.column && groupRow.column.header ?
+                groupRow.column.header :
+                (groupRow.expression ? groupRow.expression.fieldName : '') }}:
             </span>
 
             <ng-container *ngIf="dataType === 'boolean' || dataType === 'string'; else default" >