[dxwg] Pull Request: changing Link header rel="self" to rel="default"

nicholascar has just submitted a new pull request for https://github.com/w3c/dxwg:

== changing Link header rel="self" to rel="default" ==
In testing aspects of my implementation and Rob's one in devlopment, we noticed that the Link header indicating a default response with rel="self" is misleading: I had inadvertently interpreted "self" to mean the representation returned which may or may not be the default prepresentation.

Since the profile conformance and the format of the representation returned are given by `Content-Profile` and `Content-Type` respectively, they indicate the "self" result and this we would be clearer in using rel="default".

Note that in the [HTTP header mapping to the AptR model](https://w3c.github.io/dxwg/conneg-by-ap/#http-altp), we have made the fact that this rel="self" is the default indicator by mapping it to `altp:hasDefaultRepresentation`.

See https://github.com/w3c/dxwg/pull/1134