Server should partially encode Slug header values

[langsamu]

Servers MAY allow clients to suggest the URI of a resource created through POST, using the HTTP Slug header as defined in [RFC5023].

— 5.1 Resource Type Heuristics

The value supplied by a caller in the Slug header ends up an being an IRI path segment.
Because of this, implementations must deal with slug characters that would be invalid in a segment.
At least one implementation currently strips invalid characters.

I propose mandating that implementations percent encode offending characters if they honour the slug:

POST /container/
Slug: a/b

HTTP 201
Location: /container/a%2Fb

We should suggest that servers do not encode any other character but the ones illegal in path segments:

POST /container/
Slug: fancy-slug!

HTTP 201
Location: /container/fancy-slug!

The offending class of characters that should be encoded is anything outside of pchar (RFC 3986).

[kjetilk]

Yes, the topic of more sophisticated use of Slug has been discussed at some length, mainly in #128 (comment) and onwards. We decided not to tackle it yet.

However, the slugtext is just a hint to the server of what the server might want to do, and some of these details may also be left as implementation detail. Personally, I could see us assign meaning to slashes to create containers :-) Others disagree.