Add missing ignore of osm_timestamp in tagtransform_c_filter_rel_member_tags()

When reading a .osm or .o5m instead of .osm.pbf file, this caused osm2pgsql to
generate ways included in relations in planet_osm_polygons in addition to the
relation.
Get changeset attribute from .osm file, to align with .osm.pbf
Merge pull request #136 from jocelynj/osm-fixes

Unwanted way in planet_osm_polygon with test .osm