(grouping): respect customTreeAggregationFn when grouping columns

Fixes angular-ui#6402

Author: MartijnWelker

src/features/grouping/js/grouping.js

@@ -684,7 +684,13 @@
         }
 
         column.treeAggregation = { type: uiGridGroupingConstants.aggregation.COUNT, source: 'grouping' };
-        column.treeAggregationFn = uiGridTreeBaseService.nativeAggregations()[uiGridGroupingConstants.aggregation.COUNT].aggregationFn;
+
+        if ( typeof(column.colDef.customTreeAggregationFn) !== 'undefined' ) {
+          column.treeAggregationFn = column.colDef.customTreeAggregationFn;
+        } else {
+          column.treeAggregationFn = uiGridTreeBaseService.nativeAggregations()[uiGridGroupingConstants.aggregation.COUNT].aggregationFn;
+        }
+
         column.treeAggregationFinalizerFn = service.groupedFinalizerFn;
 
         grid.api.grouping.raise.groupingChanged(column);