Use lowercase for charsets #11741

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

jetty-core/jetty-http/src/main/java/org/eclipse/jetty/http/MimeTypes.java

@@ -923,6 +923,7 @@ else if (' ' != b)
     public static class ContentTypeField extends PreEncodedHttpField
     {
         private final Type _type;
+
         public ContentTypeField(MimeTypes.Type type)
         {
             this(type, type.toString());