[ADP-146] Accept-Post

This is a response only header.

Guidance

MAY use Accept-Post to tell the client it's not supported to POST with the requesting MIME type(Content-Type). SHOULD return 415 at this case.
SHOULD NOT response Accept-Post or status 415 if the client is POSTing without using Content-Type in the request header.
http
```
POST /resources
Content-Type: image/jpeg

{
    "name": "example"
}

HTTP/1.1 415 Unsupported Media Type
Accept-Post: application/json
```
TIP
💡 There's use case for a LDP server should response OPTIONS with Accept-Post. See https://www.w3.org/TR/ldp/; but we don't cover it in the API design.
http
```
GET /resources

Accept-Post: application/json, image/jpeg
```