Don't update descendant urls if slug not updated

Fixes #735

Previously, if you called Page.save with update_fields and didn't include
'slug', it would still attempt to update URL paths of descendant pages.

Wagtail 0.7 introduced a feature where it saves the latest revisions
created at date on the Page. This requires a Page.save call which used
update_fields. The bug caused update_descendant_paths to be called
anyway even though the slug wasn't committed to the database.

This caused Issue 735 because publishing a page will both save a
revision and save a page causing two hits to update_descendant_paths.
Running this method twice caused all descendant pages url_path
attributes to get mangled.