{{head-layout}} needs to be included multiple times with FastBoot

The readme says:

> As mentioned above you need to add the `{{head-layout}}` component once and only once in an application wide template

But I'm finding that I need to include `{{head-layout}}` at whichever route level I interact with the headData service in order for it to work with FastBoot.

For example, if I have `/profiles/1` which corresponds to an application route and a profile show route, both of which set data on the headData service, then I will need `{{head-layout}}` in `application.hbs` as well as `profiles/show.hbs` in order for the changes in the profile show route to be reflected after FastBoot render.

Hope that makes sense.