clear the grid when we change from singleTile mode true to false or vice versa, discussed change with @ahocevar see #821 for the details

lib/OpenLayers/Layer/Grid.js

@@ -419,6 +419,7 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
         OpenLayers.Layer.HTTPRequest.prototype.addOptions.apply(this, arguments);
         if (this.map && singleTileChanged) {
             this.initProperties();
+            this.clearGrid();
             this.tileSize = this.options.tileSize;
             this.setTileSize();
             this.moveTo(null, true);