fTools/union: fix output attribute shift

This fixes a regression (due to the Vector API change). Attributes that come from "Union" layer should be located on right places.

![union_result](https://f.cloud.github.com/assets/2672095/1061744/b760f566-11fe-11e3-937c-628eea07e3a6.png)
Image: Attribute table of Daniel's result_two_layers.shp (two_layers.tar.gz on http://hub.qgis.org/issues/8456)