AbstractFramedChannel.flushSenders invokes framePriority.frameAdded if helFrames is not empty, after having invoked framePriority.insertFrame.
For Http2FramePriority, this implies in calling generateSendFrame twice: here and here.
We can avoid the extra work and make sure we reuse the same header if the method is invoked twice, unless the frame has been deliberately cleared between the two calls.