Fix bounding box calculation

The bounding box calculation expected tuples instead of lists. This
broke the boudning box calculation of linestrings and polygons.

Change-Id: Id2cab6521226022eae77c10340f3bcc09647a453
Reviewed-on: http://review.couchbase.org/41366
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>