Custom values don't work with Rails Admin

If you define custom values for your enumeration, like:

```ruby
  enumerize :priority, in: { 
    must_read: 3,
    interesting: 2,
    not_so_interesting: 1,
    discarded: 0 }
```

Rails admin select doesn't hold the old value when editing because priority_enum would return 
```ruby
[
  ["Must read", "must_read"], 
  ["Interesting", "interesting"],
  ["Not really interesting but may be helpful", "not_so_interesting"],
  ["Discarded", "discarded"]
]
```
when 
```ruby
[
  ["Must not read", 3], 
  ["Interesting", 2], 
  ["Not really interesting but may be helpful", 1], 
  ["Discarded", 0]
]
```
is needed.