fix .cpg file contents

QGIS should write proper cpg file as possible.

Links:
- .cpg file
http://resources.arcgis.com/fr/content/kbase?fa=articleShow&d=21106

- cpg number interpretation of OGR Shapefile
https://github.com/OSGeo/gdal/blob/trunk/gdal/ogr/ogrsf_frmts/shape/ogrshapelayer.cpp#L272