protect for in loops with hasOwnProperty

lib/OpenLayers/Request.js

@@ -181,7 +181,9 @@ OpenLayers.Request = {
             config.method, url, config.async, config.user, config.password
         );
         for(var header in config.headers) {
-            request.setRequestHeader(header, config.headers[header]);
+            if (config.headers.hasOwnProperty(header)) {
+                request.setRequestHeader(header, config.headers[header]);
+            }
         }
 
         var events = this.events;