key of _enumerized_values_for_validation handled unified as Symbol

I got a strange active record validation error on save of an object with 2 enumerated fields (```:left_type``` end ```:right_type```):

The object just before save:
```
#<ObjectLink id: nil, left_id: 10, left_type: "Document", right_id: 13, right_type: "Document", note: nil, created_at: nil, updated_at: nil>
```
The validation errors:
```
["Left type can't be blank", "Right type can't be blank"]
```
This bug is caused by the fact, that the keys for the ```_enumerized_values_for_validation``` are not handled unified. It handled as Symbol but in ```lib/enumerize/activerecord.rb```in method ```write_attribute``` as String. So I got in ```_enumerized_values_for_validation``` the following:
```
{:left_type=>nil,
 "left_type"=>"Document",
 :right_type=>nil,
 "right_type"=>"Document"}
```
This pull request fixes this problem.