Use lowercase for charsets #11741

[gregw]

Fix #11741 as per the WhatTFWG recommendations, use lower case for charset names. Took the opportunity for some minor optimizations:

• use the already made HttpField instance in MimeTypes.Type rather than create a new one in the HttpParser.CACHE
• keep the MimeType.Type associated with the pre encoded Content-Type fields

[gregw]

Strange location for the ContentTypeField.

Not that strange I think. I'll add some javadoc to explain the class and I think that will make the location seam reasonable

Also, what happens if we use (in jetty-core Handler) the following?

response.getHeaders().put(HttpHeader.CONTENT_TYPE, "text/html; charset=UTF-8");

Does that produce a UTF-8? or a utf-8 on the response headers that the client sees?

We will send headers as the app sets them. If they set them with a specific case, then we will send that case. But if they use APIs to setCharset etc. then we should generate as lowercase. This PR is more about improving the parsing side, so that when we find a cached content-type, we return it as lowercase rather than the previous uppercase.

[joakime]

We will send headers as the app sets them. If they set them with a specific case, then we will send that case. But if they use APIs to setCharset etc. then we should generate as lowercase. This PR is more about improving the parsing side, so that when we find a cached content-type, we return it as lowercase rather than the previous uppercase.

Wont this cause a change in how the HttpClient parses responses too?

[gregw]

@sbordet I've made the class package protected. Can we turn this review around quickly, as I was waiting 4 days for a clean CI and now have to go back to square 1.