Overzealous GEOSGeometry sub-subclasses casts

[Claude Paroz]

Reported by Pete Flugstad in ​https://groups.google.com/forum/#!topic/geodjango/-tao0uPNYY8

When writing a Point, Polygon, etc. subclass, the parent GEOSGeometry._post_init is force casting the instance to one of the hardcoded GEOSGeometry subclass, which breaks method overriding of the subclass.

Example:

from django.contrib.gis.geos import Polygon

class ExtendedPolygon(Polygon):
    def __init__(self, point_list, data=0 ): 
        super(ExtendedPolygon, self).__init__(point_list)
        self._data = data

    def __str__(self):
        return "EXT_POLYGON - data: %d - %s" % (self._data, self.wkt )

>>> from extpoly import ExtendedPolygon
>>> points = ((-75.1, 34.3), (-74.9, 34.3), (-74.9, 34.4), (-75.1, 34.4), (-75.1, 34.3))
>>> ex = ExtendedPolygon( points, altitude=123, speed=456, direction=789 )
>>> print(ex)
    POLYGON ((-75.09999999999999 34.3, -74.90000000000001 34.3, -74.90000000000001 34.4, -75.09999999999999 34.4, -75.09999999999999 34.3))
>>> type(ex)
    <class 'django.contrib.gis.geos.polygon.Polygon'>

[Claude Paroz]

​PR

[Claude Paroz <claude@…>]

In cc9e4297:

Fixed #27557 -- Casted GEOSGeometry only when necessary

Thanks Pete Flugstad for the report, and Tim Graham for the review.