Change Content-Type?

If I register a new MIME type with Rails (say, application/hal+json) and I beat on my serializers so that they'll return valid HAL JSON... is there a way to make `ActiveModel::Serializers` report that new MIME type in the `Content-Type` header?

At first, I was hoping that something like this in the controller would work:

``` ruby
render hal: @posts, serializer: PostsSerializer
```

But then Rails::API goes looking for a *template*, and ignores my `serializer` option.

If this isn't something that's currently possible, is this something folks would be interested? If people are, I could see about taking a crack.