Align terminology with RFC7231. See OAI#2476

This PR distinguish between Content-Encoding HTTP header field, which is an ordered list of content-codings
 and Content-Transfer-Encoding which is a multipart header (see https://tools.ietf.org/html/rfc7578#section-4.5).

Author: ioggstream

versions/3.1.0.md

@@ -1448,7 +1448,7 @@ application/json:
 
 In contrast with the 2.0 specification, `file` input/output content in OpenAPI is described with the same semantics as any other schema type.
 
-In contrast with the 3.0 specification, the `format` keyword has no effect on the content-encoding of the schema. JSON Schema offers a `contentEncoding` keyword, which may be used to specify the `Content-Encoding` for the schema. The `contentEncoding` keyword supports all encodings defined in [RFC4648](https://tools.ietf.org/html/rfc4648), including "base64" and "base64url", as well as "quoted-printable" from [RFC2045](https://tools.ietf.org/html/rfc2045#section-6.7). The encoding specified by the `contentEncoding` keyword is independent of an encoding specified by the `Content-Type` header in the request or response or metadata of a multipart body -- when both are present, the encoding specified in the `contentEncoding` is applied first and then the encoding specified in the `Content-Type` header.
+In contrast with the 3.0 specification, the `format` keyword has no effect on the content-encoding of the schema. JSON Schema offers a `contentEncoding` keyword, which may be used to specify the `Content-Encoding` for the schema. The `contentEncoding` keyword supports all encodings defined in [RFC4648](https://tools.ietf.org/html/rfc4648), including "base64" and "base64url", as well as "quoted-printable" from [RFC2045](https://tools.ietf.org/html/rfc2045#section-6.7). The encoding specified by the `contentEncoding` keyword is independent of the eventual content-codings specified by the `Content-Encoding` HTTP header field in the request or response, or in the `Content-Transfer-Encoding` of a multipart body -- when both are present, the encoding specified in the `contentEncoding` is applied first and then the content-codings specified in the `Content-Encoding` HTTP header field.
 
 JSON Schema also offers a `contentMediaType` keyword.  However, when the media type is already specified by the Media Type Object's key, or by the `contentType` field of an [Encoding Object](#encodingObject), the `contentMediaType` keyword SHALL be ignored if present.