Added option to overwrite existing stored credentials.

Added an option to django_orm.Storage locked_put method to allow for
overwriting an existing credential record rather than the creation of a
new one.

oauth2client/django_orm.py

@@ -116,14 +116,19 @@ class Storage(BaseStorage):
         credential.set_store(self)
     return credential
 
-  def locked_put(self, credentials):
+  def locked_put(self, credentials, overwrite=False):
     """Write a Credentials to the datastore.
 
     Args:
       credentials: Credentials, the credentials to store.
     """
     args = {self.key_name: self.key_value}
-    entity = self.model_class(**args)
+    
+    if overwrite: 
+      entity, is_new = self.model_class.objects.get_or_create(**args)
+    else: 
+      entity = self.model_class(**args)
+    
     setattr(entity, self.property_name, credentials)
     entity.save()