Bug correction : mix of variables/values with the union function

[llaporte39]

The union function mix some variables, depending of the order of the variables. Sometimes, the values will be inverted bewteen two columns.

For example, if you have two datasets with only the identifier in the variables, and you try this :

ds1 := ds1 [calc TOTO := 1];
ds1 := ds1 [calc TITI := 2];

ds2 := ds2 [calc TITI := 3];
ds2 := ds2 [calc TOTO := 4];

ds3 := union(ds1, ds2);
ds4 := union(ds2, ds1);

the two datasets ds3 and ds4 don't have the same values, the variables TOTO and TITI are inversed.