Support basic Entity API default_revision property on nodes

For fields like Field Collection that define an entity, support revisions via Entity API, and need to save their own entity when the host node is saved, there is no way to determine whether the host node is the default revision (published revision) at $entity->save() time without explicitly calling workbench_moderation_* functions.

This very simple two-line patch sets a $node->default_revision property, so an embedded revisionable entity calling entity_revision_is_default() on its host node gets the correct value, allowing it to determine how to save its entity revision.