Allow duplicate Server headers when proxying response

See discussion on commit c9b022b5e0

If we overwrite the Server header, it becomes difficult/impossible to
know from the client whether the request was proxied through Caddy.

caddyhttp/proxy/reverseproxy.go

@@ -413,9 +413,13 @@ func copyHeader(dst, src http.Header) {
 			if _, shouldSkip := skipHeaders[k]; shouldSkip {
 				continue
 			}
-			// otherwise, overwrite
+			// otherwise, overwrite to avoid duplicated fields that can be
+			// problematic (see issue #1086) -- however, allow duplicate
+			// Server fields so we can see the reality of the proxying.
+			if k != "Server" {
 				dst.Del(k)
 			}
+		}
 		for _, v := range vv {
 			dst.Add(k, v)
 		}