GeometryCollection should be an allowed type for GeometryCollection

[Jacob Walls]

The multivalued geometry fields, e.g. MultiPolygon, define which classes they can contain, e.g. Polygon but not Point, via the private attribute _allowed.

GeometryCollection is generic, so it contains all the others:

# Setting the allowed types here since GeometryCollection is defined before
# its subclasses.
GeometryCollection._allowed = (
    Point,
    LineString,
    LinearRing,
    Polygon,
    MultiPoint,
    MultiLineString,
    MultiPolygon,
)

However, this list should also contain itself, since GeometryCollections can be nested.

In other words, this should work:

django/contrib/gis/geos/collections.py

@@ -123 +123 @@
     MultiPoint,
     MultiLineString,
     MultiPolygon,
+    GeometryCollection,
 )

tests/gis_tests/geos_tests/test_geos.py

@@ -1347 +1347 @@
         # And, they should be equal.
         self.assertEqual(gc1, gc2)
 
+        # Should also construct ok from GeometryCollection.
+        gc3 = GeometryCollection(gc2)
+        self.assertEqual(gc1, gc3[0])
+
     def test_gdal(self):
         "Testing `ogr` and `srs` properties."
         g1 = fromstr("POINT(5 23)")