Give our requests the popular but non-standard X-Requested-With

header (r=crschmidt, closes #3491).

lib/OpenLayers/Request.js

@@ -122,6 +122,25 @@ OpenLayers.Request = {
             {proxy: OpenLayers.ProxyHost}
         );
         config = OpenLayers.Util.applyDefaults(config, defaultConfig);
+        
+        // Always set the "X-Requested-With" header to signal that this request
+        // was issued through the XHR-object. Since header keys are case 
+        // insensitive and we want to allow overriding of the "X-Requested-With"
+        // header through the user we cannot use applyDefaults, but have to 
+        // check manually whether we were called with a "X-Requested-With"
+        // header.
+        var customRequestedWithHeader = false;
+        for(headerKey in config.headers) {
+            if (config.headers.hasOwnProperty( headerKey )) {
+                if (headerKey.toLowerCase() === 'x-requested-with') {
+                    customRequestedWithHeader = true;
+                }
+            }
+        }
+        if (customRequestedWithHeader === false) {
+            // we did not have a custom "X-Requested-With" header
+            config.headers['X-Requested-With'] = 'XMLHttpRequest';
+        }
 
         // create request, open, and set headers
         var request = new OpenLayers.Request.XMLHttpRequest();