[FIX] web: returns right deferred when loading same xml twice in a row
[FIX] web: returns right deferred when loading same xml twice in a row

Since commit 417a664f16abc67bee40a0067103f9cf2c412aed:
The `ajax.loadXML` function has been refactored and improved so that
multiple calls to the function with the same URL only loads it once
and so that the deferred which is returned really indicates the status
of loading this particular URL.

Unfortunately, there was a mistake: when loading the same URL twice,
the second call always returned an already resolved deferred if the
first call to `ajax.loadXML` was done with this URL.

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