fix get_format return policy
fix get_format return policy

davenquinn/python-mapnik,tomhughes/python-mapnik,mapnik/python-mapnik,garnertb/python-mapnik,tomhughes/python-mapnik,garnertb/python-mapnik,mapycz/python-mapnik,davenquinn/python-mapnik,mapycz/python-mapnik,davenquinn/python-mapnik,mapnik/python-mapnik,mapnik/python-mapnik,tomhughes/python-mapnik,garnertb/python-mapnik