MeidaType's subtype with suffix should be a subset of a subtype without suffix

[kichalla]

Is this a Bug or Feature request?:

Bug

Steps to reproduce (preferably a link to a GitHub repo with a repro project):

Add the following 2 tests to the IsSubsetOf_ReturnsTrueWhenExpected test in Microsoft.AspNetCore.Mvc.Formatters.MediaTypeTest and they fail

[InlineData("application/json", "application/vnd.restful+json")]
[InlineData("application/json", "application/problem+json")]

Description of the problem:

According to https://tools.ietf.org/html/rfc6839#section-2, content like application/problem+json should be able to be processed as though its application\json too, but currently we don't consider application/problem+json as a subtype.

Version of Microsoft.AspNetCore.Mvc or Microsoft.AspNetCore.App or Microsoft.AspNetCore.All:

2.1.3 and latest

[Eilon]

Tagging @dougbu and @jkotalik who I think both have a great deal of knowledge on this matter.

[dougbu]

@kichalla and I discussed this, reviewed the spec(s), and I agree it's a bug.

[jkotalik]

I can take this one if we just need to get it in preview-1. Would this apply to both MediaTypeHeaderValue and MVC's MediaType?

[dougbu]

@jkotalik dotnet/aspnetcore#3362 covers MediaTypeHeaderValue though the fixes should be the same or close to it. (The main difference that I remember in the relevant code is MediaType has more comments.)

[jkotalik]

I'll go ahead and assign myself to both. I'll crunch them out when I have a spare moment in the next week.