notebooks should always have one checkpoint

closes #4396

IPython/html/services/notebooks/filenbmanager.py

@@ -222,6 +222,10 @@ class FileNotebookManager(NotebookManager):
 
         if 'content' not in model:
             raise web.HTTPError(400, u'No notebook JSON data provided')
+        
+        # One checkpoint should always exist
+        if self.notebook_exists(name, path) and not self.list_checkpoints(name, path):
+            self.create_checkpoint(name, path)
 
         new_path = model.get('path', path).strip('/')
         new_name = model.get('name', name)

IPython/html/services/notebooks/nbmanager.py

@@ -117,7 +117,7 @@ class NotebookManager(LoggingConfigurable):
         raise NotImplementedError('must be implemented in a subclass')
 
     def create_notebook_model(self, model=None, path=''):
-        """Create a new untitled notebook and return its model with no content."""
+        """Create a new notebook and return its model with no content."""
         path = path.strip('/')
         if model is None:
             model = {}