Shapely cannot compare a geometry with None

[elemoine]

In our Geometry TypeEngine we have this compare_values method for comparing geometries:

    def compare_values(self, x, y):
        return x.equals(y)

(x and y are Shapely geometries)

But Shapely gives an error when comparing a geometry with None (geometry.equals(None)). So I suggest that we guard against that and change our code to:

    def compare_values(self, x, y):
        t = (x, y)
        if None not in t:
            return x.equals(y)
        return t == (None, None)

[elemoine]

Tests continue to pass. Please review.

[aabt]

Looks good to me, thanks for the patch. Please commit.

[elemoine]

(In [3453]) Shapely cannot compare a geometry to None, work around that, r=aabt (closes #567)