Fix pickling/unpickling of Dataset instances.
Fix pickling/unpickling of Dataset instances.

zhengzhihust/tablib,steveb/tablib,enzochiau/tablib,tommyanthony/tablib,kennethreitz/tablib,bopo/tablib,godfreyy/tablib,leohmoraes/tablib,bopo/tablib,mloesch/tablib,screwt/tablib,klothe/tablib,leohmoraes/tablib,alex/tablib,durden/tablib,whix/tablib,godfreyy/tablib,steveb/tablib,klothe/tablib,whix/tablib,mloesch/tablib,zhengzhihust/tablib,screwt/tablib,enzochiau/tablib