GEOS-6709 fix import transforms, add tests

The modified transforms were using the wrong argument for obtaining
the attribute to transform. This was working for JDBC Feature
implementations but not for others.
Merge pull request #777 from ischneider/GEOS-6709

GEOS-6709 fix import transforms, add tests
Merge branch 'master' of git://github.com/geoserver/geoserver