Calculate zoom level from serverResolutions.

The previous way of hard-coding an offset of 1 causes an incorrect attribution for layers with a custom maxResolution.

lib/OpenLayers/Layer/Bing.js

@@ -223,7 +223,8 @@ OpenLayers.Layer.Bing = OpenLayers.Class(OpenLayers.Layer.XYZ, {
             this.map.getProjectionObject(),
             new OpenLayers.Projection("EPSG:4326")
         );
-        var providers = res.imageryProviders, zoom = this.map.getZoom() + 1,
+        var providers = res.imageryProviders,
+            zoom = this.serverResolutions.indexOf(this.getServerResolution()),
             copyrights = "", provider, i, ii, j, jj, bbox, coverage;
         for (i=0,ii=providers.length; i<ii; ++i) {
             provider = providers[i];