Use URI for X-Powered-By value

[The following is not strictly a node-solid-server issue but I think it can go a long way in setting it as a practice that other Solid server implementations or variations can follow suite. Eventually, this 'feature' can bubble up to the solid-spec - given that it is a common practice (aka sufficiently "incubated")]

The X-Powered-By header value eg solid-server can be more specific/unique. After all, which Solid server or which version? So, Solid servers should announce a URI as a value instead which can potentially be self-descriptive when dereferenced ie. another agent can find out about its capabilities, what specs it implements, making use of vocabs like DOAP stuff. This is beyond content or time negotiation etc.

Edit: granted that the string "solid-server" can mean that it is an implementation of the Solid spec, but that may be under the assumption that the server conforms to the latest state of the spec. While the clients could make that assumption, they can be mislead given what the server actually performs. So, the reality is that, there are indeed different versions of the application, as well as its conformance to the specs. All of that information and more can be discoverable through the URI.

eg curl -I https://linkedresearch.org/ldn/tests/ uses X-Powered-By: https://github.com/csarven/mayktso but it can also point to something more granular and actually return RDF.

En el issue no se identifica ningún archivo de implementación ni ninguna prueba. Empieza localizando dónde node-solid-server establece la cabecera de respuesta X-Powered-By y, a continuación, revisa los comentarios existentes y las convenciones de Solid-server antes de decidir qué URI y representación son aceptables. Se considera terminado cuando la cabecera expone de forma coherente la URI autod descriptiva acordada y su comportamiento está cubierto por pruebas.