ServerCache should respect Vary headers

The ServerCache enabled by the ServerCacheFeature does not respect the Vary header, which tells the headers that may modify the response for the same URI. E.g., this is a correct request/response exchange:

GET /hello HTTP/1.1
Accept: text/plain
Accept-Language: en

HTTP/1.1 200 OK
Cache-Control: max-age=60
Content-Type: text/plain
Vary: Accept
Vary: Accept-Language

Hello!

GET /hello HTTP/1.1
Accept: text/plain
Accept-Language: it

HTTP/1.1 200 OK
Cache-Control: max-age=60
Content-Type: text/plain
Vary: Accept
Vary: Accept-Language

Ciao!

With the ServerCacheFeature, the last response would contain Hello! (i.e. cached from the previous request), instead of Ciao!.
When caching responses, the ServerCache should use all Vary headers in it and use them to match possible cached results.