Multiple default groups shown in Stats

There are two `default` groups in this list - in the 2nd and 6th positions.

When clicking the one in the 6th position, then the list for the one in the 2nd position appear in the panel below this list i.e. I can't access details of the requests that had 163 bytes.

There appears to be a couple of instances ([1](https://github.com/Automattic/wp-memcached/blob/811243804a892a4609a4581d9479fa80c4e4ac8d/object-cache.php#L642-L644), [2](https://github.com/Automattic/wp-memcached/blob/811243804a892a4609a4581d9479fa80c4e4ac8d/object-cache.php#L658-L660)) where a missing group name gets set as `default`, and it's [the group name that is used for the toggle](https://github.com/Automattic/wp-memcached/blob/811243804a892a4609a4581d9479fa80c4e4ac8d/object-cache.php#L651).

Start in object-cache.php at the referenced lines around 642-660, then trace how the group name is used for the stats toggle. Reproduce the duplicate default entries if possible; done means each group has distinct, correctly selected details in the panel, including the requests with 163 bytes.

Written by the indexing model from the issue text.