Do not cache data from aborted tile loads

This also results in a simplified cache method that can more easily be
overridden for use with other storage providers.

lib/OpenLayers/Layer/Grid.js

@@ -262,8 +262,9 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
      *     loaded, as a means of progress update to listeners.
      *     listeners can access 'numLoadingTiles' if they wish to keep
      *     track of the loading progress. Listeners are called with an object
-     *     with a tile property as first argument, making the loded tile
-     *     available to the listener.
+     *     with a 'tile' property as first argument, making the loded tile
+     *     available to the listener, and an 'aborted' property, which will be
+     *     true when loading was aborted and no tile data is available.
      * tileerror - Triggered before the tileloaded event (i.e. when the tile is
      *     still hidden) if a tile failed to load. Listeners receive an object
      *     as first argument, which has a tile property that references the
@@ -1090,9 +1091,12 @@ OpenLayers.Layer.Grid = OpenLayers.Class(OpenLayers.Layer.HTTPRequest, {
             this.numLoadingTiles++;
         };
       
-        tile.onLoadEnd = function() {
+        tile.onLoadEnd = function(evt) {
             this.numLoadingTiles--;
-            this.events.triggerEvent("tileloaded", {tile: tile});
+            this.events.triggerEvent("tileloaded", {
+                tile: tile,
+                aborted: evt.type === "unload"
+            });
             //if that was the last tile, then trigger a 'loadend' on the layer
             if (this.tileQueue.length === 0 && this.numLoadingTiles === 0) {
                 this.loading = false;