Fix bulk-delete on all-doc-page COUCHDB-2462

On Couch 1.6 `/_bulk_docs` returned `ok: true` when the request was an
success. On 2.0 the ok property is missing:

Example success message on 2.0:
```
[{"id":"Brocket","rev":"6-b6fa9e703c3eb92aa9c3a49cedf1e8c2"}]
```

Example error message on 2.0:
```
[{"id":"Blaggie-System","error":"conflict",
  "reason":"Document update conflict."}]
```

Steps to reproduce the error in Fauxton:

 - select 1 document in the all_docs screen
 - click the trash icon to delete it
 - click ok to delete it
 - you are able to click the trash icon again and if you do, you
   get an error

Closes COUCHDB-2462