Java11 Http2Client loses response header value order and duplicate occurrences #3636

The Java11 client converts each list of response header values to a HashSet in castMapCollectType, losing repeated occurrences and their received order. Both synchronous and asynchronous callers see the changed values through Response.headers().

I reproduced this on current master (fee42b4) against a real local MockWebServer, adding each value as a separate header line. Six cases fail across sync/async execution: ordered distinct values, repeated identical Set-Cookie values, and mixed distinct/duplicate values. Both single-value controls pass; there are no test errors or skips.

For example, sending two identical Set-Cookie: session=abc; Path=/ lines returns only one value in response.headers().get("set-cookie").

JDK HttpHeaders preserves each occurrence and the received order of response header values. Feign's core header copy and the OkHttp/Apache HTTP5 adapters also preserve the lists. I am preparing a small list-preserving conversion fix with the real HTTP regression tests.