[fixup] minKey & maxKey toJSON()

Just to be consistent with what you see in the mongo shell.

say you're querying `db('config').chunks.find({}, {min: 1, max: 1})` to get a chunk's keyspace.  instead of 
```
keyspace: {"_id":{"_bsontype":"MinKey"}} → {"_id":{"_bsontype":"MaxKey"}}
```

be more human

```
keyspace: {"_id":{"$minxKey":1}} → {"_id":{"$maxKey":1}}
```