Update data_format on data change
Before that patch, and in the context of an update thought a form, the data, and format as resent.
If the user changes the data, he usually expects the format attribute to be automatically updated too. But it was not.
With this patch, if the data is changed, and the data_format is the same as before, we assume it has not been updated by the user, and that it should be recomputed. On the other hand, if the value is different than the previous one, we assume it has been changed by the user and we keep this value.
Before that patch, the suggested test fails as follow
