Work around for geotools shapefile datastore missbehaving if paging is used.

The switch to geotools 10.5 implied the use of the new shapefile
datastore which is based on ContentDataStore and supports paging
out of the box through the org.geotools.data.sort.SortedFeatureReader,
but it is forcing to dump all the shapefile features to a file before
starting the import and it has a bug where some features may be missing
even if Query.offset == 0.
Make shp export honor the name of the attributes in the output schema

Before this change, exporting a feature with a geometry name other than
"the_geom" (the default used by the geotools for a shapefile) resulted
in an empty .shp file