Check for awaitable instead of coroutine in stream glue

[anuraaga]

The into_stream glue code provides backpressure on the wrapped async generator when its internal buffer is full. This is done by returning a future that resolves after the buffer flushes and the item sent. But a future is not a coroutine so the iscoroutine check currently used always returns False and the future is never awaited. This causes the generator to continue to be pulled non-stop until completion, at which point the glue's tx will be closed even though there are still pending sends, causing them to be missed.

The correct check is isawaitable to detect asyncio.Future.

[davidhewitt]

Thanks, this makes sense to me, I verified both that I agree with the test and that the fix does indeed resolve the test.