Don't issue flow control credit for streams with known size

A stream's size is only known after we've issued at least that much
flow control credit, so the peer necessarily won't need any more. Our
logic to schedule credit issuing already uses similar judgement, so
this brings the last-minute sanity check into alignment.

quinn-proto/src/connection/streams/state.rs

@@ -381,7 +381,7 @@ impl StreamsState {
                 Some(x) => x,
                 None => continue,
             };
-            if !rs.is_receiving() {
+            if !rs.receiving_unknown_size() {
                 continue;
             }
             retransmits.get_or_create().max_stream_data.insert(id);