fix(clearThisRowInvisible) not using parameter reason

Not sure if intended but clearThisRowInvisible is not using the parameter "reason" when deleting the "invisible reason" but it always deletes the hardcoded parameter "user". 
This shouldn't change any other behavior since ng-grid internal functions call this function only with the parameter "user" but it will fix any error if someone was directly using this function with a custom invisible reason

Author: valepu

src/js/core/factories/GridRow.js

@@ -176,7 +176,7 @@ angular.module('ui.grid')
    */
   GridRow.prototype.clearThisRowInvisible = function ( reason, fromRowsProcessor ) {
     if (typeof(this.invisibleReason) !== 'undefined' ) {
-      delete this.invisibleReason.user;
+      delete this.invisibleReason[reason];
     }
     this.evaluateRowVisibility( fromRowsProcessor );
   };
@@ -216,4 +216,4 @@ angular.module('ui.grid')
   return GridRow;
 }]);
 
-})();
+})();