(#1973) - Adapted the destroy tests

we should always expect an info object back
(#1973) - on Pouchdb.destroy call callback with response

if the given respone is null, we replace it with {ok: true} to
mimic couch's behaviour
(#1973) - resolve the destroy of leveldb with an object

by default leveldown will call the given callback without an
argument in case of success. We instead call it with {ok: true}