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

There was a problem with the new `ajax.loadXML` implementation
introduced with 417a664f16abc67bee40a0067103f9cf2c412aed

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).

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