Clarify support of UTF-8 and other encodings (#232)

* Clarify support of UTF-8 and other encodings

See #229

* Update GraphQLOverHTTP.md

* Update INTERESTED_DEVELOPERS.md

* Update GraphQLOverHTTP.md

* Update spec/GraphQLOverHTTP.md

Co-authored-by: Benjie <benjie@jemjie.com>

* Update GraphQLOverHTTP.md

* Update GraphQLOverHTTP.md

* Update spec/GraphQLOverHTTP.md

* Update spec/GraphQLOverHTTP.md

Co-authored-by: Benjie <benjie@jemjie.com>

* Update spec/GraphQLOverHTTP.md

Co-authored-by: Benjie <benjie@jemjie.com>

* Update spec/GraphQLOverHTTP.md

Co-authored-by: Benjie <benjie@jemjie.com>

* Apply suggestions from code review

* Update formatting

---------

Co-authored-by: Benjie <benjie@jemjie.com>

Author: Shane32

spec/GraphQLOverHTTP.md

@@ -193,10 +193,11 @@ And for a _GraphQL response_:
 For details of the shapes of these JSON payloads, please see
 [Request](#sec-Request) and [Response](#sec-Response).
 
-If the media type in a `Content-Type` or `Accept` header includes encoding
-information, then the encoding MUST be `utf-8` (e.g.
-`Content-Type: application/graphql-response+json; charset=utf-8`). If encoding
-information is not included then `utf-8` MUST be assumed.
+If the media type in a `Content-Type` or `Accept` header does not include
+encoding information and matches one of the officially recognized GraphQL media
+types, then `utf-8` MUST be assumed (e.g. for header
+`Content-Type: application/graphql-response+json`, UTF-8 encoding would be
+assumed).
 
 # Request
 
@@ -328,7 +329,10 @@ A client MUST indicate the media type of a request body using the `Content-Type`
 header as specified in [RFC7231](https://datatracker.ietf.org/doc/html/rfc7231).
 
 A server MUST support POST requests encoded with the `application/json` media
-type (as indicated by the `Content-Type` header).
+type (as indicated by the `Content-Type` header) encoded with UTF-8.
+
+For POST requests using an officially recognized GraphQL `Content-Type` without
+indicating an encoding, the server MUST assume the encoding is `utf-8`.
 
 If the client does not supply a `Content-Type` header with a POST request, the
 server SHOULD reject the request using the appropriate `4xx` status code.
@@ -339,7 +343,7 @@ they wish when none is supplied, with the understanding that parsing the request
 may fail.
 
 A server MAY support POST requests encoded with and/or accepting other media
-types.
+types or encodings.
 
 If a client does not know the media types the server supports then it SHOULD
 encode the request body in JSON (i.e. with `Content-Type: application/json`).
@@ -415,7 +419,8 @@ The body of the server's response MUST follow the requirements for a
 [GraphQL response](#sec-Response), encoded directly in the chosen media type.
 
 A server MUST indicate the media type of the response with a `Content-Type`
-header.
+header, and SHOULD indicate the encoding (e.g.
+`application/graphql-response+json; charset=utf-8`).
 
 If an `Accept` header is provided, the server MUST respect the given `Accept`
 header and attempt to encode the response in the highest priority media type
@@ -451,7 +456,7 @@ January 2025.
 
 From 1st January 2025 (`2025-01-01T00:00:00Z`), a server MUST support requests
 which accept the `application/graphql-response+json` media type (as indicated by
-the `Accept` header).
+the `Accept` header) using the UTF-8 encoding.
 
 Before 1st January 2025 (`2025-01-01T00:00:00Z`), if the client does not supply
 an `Accept` header, the server SHOULD treat the request as if it had