Use protocol-relative URL for Bing metadata.

The [URI standard](http://tools.ietf.org/html/rfc3986#section-4.2) allows for URI without a scheme.  Where http is used to load the app, using a schemeless URL for the Bing metadata loads a script using http - and the same goes for https.

lib/OpenLayers/Layer/Bing.js

@@ -145,7 +145,7 @@ OpenLayers.Layer.Bing = OpenLayers.Class(OpenLayers.Layer.XYZ, {
             jsonp: this._callbackId,
             include: "ImageryProviders"
         }, this.metadataParams);
-        var url = "http://dev.virtualearth.net/REST/v1/Imagery/Metadata/" +
+        var url = "//dev.virtualearth.net/REST/v1/Imagery/Metadata/" +
             this.type + "?" + OpenLayers.Util.getParameterString(params);
         var script = document.createElement("script");
         script.type = "text/javascript";