Fix issue with deleting Releaf::Node, when content object can't be constantized

If calss that was referenced by Releaf::Node was deleted from project.
Destroying such node would be impossible, because NameError exception
would be raised by rails (trying to destroy related object),

In case this exception is raised we set content_id and content_type to
nil to eliminate polymorphic association and save node without
validating it (otherwise rails somehow tries to destroy related object.
ie ignores unsaved changed).
After record is saved we call destroy again.