Respect rings closure of polygon while reading WKT input

[Mateusz Loskot]

This task is reverse of #9217.

If model of a polygon specifies non-closed rings (first point is not repeated at last position), then this specification should be respected while consuming polygon encoded in OGC WKT format in which all rings are always closed.

For example, the point number assertion below should hold:

using namespace boost::geometry;

typedef model::point< double, 2, boost::geometry::cs::cartesian > Point;
typedef model::polygon< Point, false, false > Polygon;

Polygon p;
read_wkt("POLYGON((1 1, 5 1, 5 5, 1 5, 1 1))", p);

assert(p.outer().size() == 4);