[feature] Flatten polymorphic id on has_one associations

Specify the association like this:
```ruby
class MessageSerializer
    attributes :content
    has_one :attachment, polymorphic: true, flatten: true
end
```

Get the JSON value like this.
```json
{
    "id" : 1,
    "content" : "hello",
    "attachment_id" : 1,
    "attachment_type" : "link"
}
```