Enum.keys returns strings

From your example

``` ruby
class User < ActiveRecord::Base
  as_enum :gender, female: 1, male: 0
end

# expected
user.gender == :female
user.genders.key(1) == :female

# but
user.genders.keys == ["male", "female"]
```

ie. the enumerated values always appear as symbols everywhere except when calling `keys`.
