Lazily-populated caches on a ``WCS`` (such as the internal
``world_axis_object_components``/``world_axis_object_classes`` cache) are no
longer included in the pickled state. They are regenerated on demand after
unpickling, which keeps pickling robust even when a cache entry holds a
non-picklable value.

Fixed a bug where the ``preserve_units`` option passed to the ``WCS``
constructor was silently reset to ``False`` when a ``WCS`` object was pickled
and unpickled.
