[FIX] web: resolve ajax.loadXML deferred at the proper time

There was a problem with the new `ajax.loadXML` implementation
introduced with 417a664f16

Indeed the following case was buggy:

```

ajax.loadXML('URL1', qweb);
ajax.loadXML().then(function () {
   ajax.loadXML('URL2', qweb);
});

```

With this code, 'URL2' is scheduled to be loaded when 'URL1' is fully
loaded. The problem is that the deferred returned by the call to
`ajax.loadXML` without argument was resolved before the internal
`isLoading` variable was reset to `false`. So, the loading of 'URL2'
was indeed scheduled when 'URL1' was fully loaded but its loading
never started because the `ajax.loadXML` loading loop was still marked
as running (so it was not started again as it should have been).

addons/web/static/src/js/core/ajax.js

@@ -426,8 +426,8 @@ var loadXML = (function () {
             } else {
                 // There is nothing to load anymore, so resolve the
                 // "all the calls" deferred
-                allLoadingsDef.resolve();
                 isLoading = false;
+                allLoadingsDef.resolve();
             }
         }
     };