fix: Wrong sort priorities 4653 and 4196

4653: sort Priority 1 is presented as 0 in the sort object. It shows true for !column.sort.priority so it should be checked with column.sort.priority === undefined.
4196: add function to decrease higher priorities if a column sort with lower priority is removed.

Added test cases to verify behaviour.

src/js/core/factories/Grid.js

@@ -1979,7 +1979,7 @@ angular.module('ui.grid')
       // Get the actual priority since there may be columns which have suppressRemoveSort set
       column.sort.priority = self.getNextColumnSortPriority();
     }
-    else if (!column.sort.priority){
+    else if (column.sort.priority === undefined){
       column.sort.priority = self.getNextColumnSortPriority();
     }
 
@@ -1997,7 +1997,7 @@ angular.module('ui.grid')
       if (column.sortDirectionCycle[i]) {
         column.sort.direction = column.sortDirectionCycle[i];
       } else {
-        column.sort = {};
+        removeSortOfColumn(column, self);
       }
     }
     else {
@@ -2009,6 +2009,18 @@ angular.module('ui.grid')
     return $q.when(column);
   };
 
+  var removeSortOfColumn = function removeSortOfColumn(column, grid) {
+    //Decrease priority for every col where priority is higher than the removed sort's priority.
+    grid.columns.forEach(function (col) {
+      if (col.sort && col.sort.priority !== undefined && col.sort.priority > column.sort.priority) {
+        col.sort.priority -= 1;
+      }
+    });
+
+    //Remove sort
+    column.sort = {};
+  };
+
   /**
    * communicate to outside world that we are done with initial rendering
    */

test/unit/core/factories/Grid.spec.js

@@ -810,6 +810,32 @@ describe('Grid factory', function () {
       expect( column.sort.direction ).toEqual(uiGridConstants.ASC);
       expect( column.sort.priority ).toEqual(0);
     });
+
+    it( 'if two column has sort 1 and 2 on the ui which is 0 and 1 in the sort object and the sort change for the first do not change the priority', function() {
+      var priorColumn1 = new GridColumn({ name: 'a', sort: { direction: uiGridConstants.ASC, priority: 0 } });
+      var priorColumn2 = new GridColumn({ name: 'b', sort: { direction: uiGridConstants.ASC, priority: 1 } });
+      grid.columns.push( priorColumn1 );
+      grid.columns.push( priorColumn2 );
+
+      grid.sortColumn( priorColumn1, true );
+
+      expect( priorColumn1.sort ).toEqual({ direction: uiGridConstants.DESC, priority: 0});
+    });
+
+    it( 'if three column has sort 1,2 and 3 on the ui which is 0,1 and 2 in the sort object and the sort removed for the second decrease priority for the third but do not change for the first', function() {
+      var priorColumn1 = new GridColumn({ name: 'a', sort: { direction: uiGridConstants.ASC, priority: 0 } });
+      var priorColumn2 = new GridColumn({ name: 'b', sort: { direction: uiGridConstants.DESC, priority: 1 } });
+      var priorColumn3 = new GridColumn({ name: 'c', sort: { direction: uiGridConstants.ASC, priority: 2 } });
+      grid.columns.push( priorColumn1 );
+      grid.columns.push( priorColumn2 );
+      grid.columns.push( priorColumn3 );
+
+      grid.sortColumn( priorColumn2, true );
+
+      expect( priorColumn1.sort ).toEqual({ direction: uiGridConstants.ASC, priority: 0 });
+      expect( priorColumn2.sort ).toEqual({ });
+      expect( priorColumn3.sort ).toEqual({ direction: uiGridConstants.ASC, priority: 1 });
+    });
   });