Support for RFC6839

Additional media types (+json)

What?

Support RFC6839 allowing formatters for additional media types structured with syntax suffixes, like +json.

Valid examples are: application/calendar+json, application/problem+json, application/geo+json, etc (the full list can be seen on mime-db)...

Why?

Right now if you try to follow RFC7807 and send back a Content-Type application/problem+json, it won't find the correct formatter (which is json) and will override the content-type to application/octet-stream.

Example API

server.get('/', function (req, res, next) {
  res.contentType = 'problem+json';
  res.send({
    "type": "https://example.com/probs/out-of-credit",
    "title": "You do not have enough credit.",
    "detail": "Your current balance is 30, but that costs 50." 
  });
});

Are you willing and able to implement this?

I'm converting PR #1786 from RFC7807 to RFC6839 and we can tackle RFC7807 on a separate issue.

Start by tracing how the server.get example reaches res.contentType and res.send, then locate the formatter selection for media types. Verify behavior with application/problem+json and the other RFC6839 +json examples; done means the json formatter is selected without replacing the declared content type with application/octet-stream.