I have found that adding headers does, in fact, add the headers unless there is also a host defined. I am using the latest version of Ember Data.
Looking at the RESTadapter the ajaxOptions method is what actually adds the headers. This is called by the ajax method. None of this seems to rely on the existence of a host.
@mchan do you also have a host defined? @jamalsoueidan does this shine some light? I’m trying to work this out still.