Fixed `ArgumentError: unsupported attribute type DateTime` on document reload

`Dynamoid::Document` is raising an `ArgumentError: unsupported attribute type DateTime` for models that uses `:datetime` for the `range` attribute definition. Here is an example:

```ruby
class Message
  include Dynamoid::Document
  
  table name: :messages, key: :message_id
  range :created_at, :datetime

  field :text, :string
end

message = Message.new(text: "Ah, I'm raising an ArgumentError!")
message.save.reload
# => <ArgumentError: unsupported attribute type DateTime>
```

I fixed that problem by using `#dump_field` in the `reload` method on the `range_value`.