h3: set stream priority after headers buffered

send_headers() can fail if there is not enough stream capacity to buffer
encoded headers and applications are expected to try again later.
Previously, send_response_with_priority() would update
the stream priority prior to send_headers(), which might surprise
some types of applications relying on the writable() iterator.

This change swaps the ordering around so that priority is only changed
after headers have been successfully buffered.

quiche/src/h3/mod.rs

@@ -1164,6 +1164,8 @@ impl Connection {
             return Err(Error::FrameUnexpected);
         }
 
+        self.send_headers(conn, stream_id, headers, fin)?;
+
         // Clamp and shift urgency into quiche-priority space
         let urgency = priority
             .urgency
@@ -1172,8 +1174,6 @@ impl Connection {
 
         conn.stream_priority(stream_id, urgency, priority.incremental)?;
 
-        self.send_headers(conn, stream_id, headers, fin)?;
-
         Ok(())
     }