Fix bug 1241 by calling acknowledge_received_data

[Yaakov-Belch]

Fix bug #1241: asyncio-server.py example is not working correctly.

The bug was caused by not updating the Flow Control Window when receiving data. The necessary steps are documented here:

• https://python-hyper.org/projects/hyper-h2/en/stable/advanced-usage.html#flow-control
• https://python-hyper.org/projects/hyper-h2/en/stable/advanced-usage.html#auto-flow-control

In short, all that needs to be done is:

• Call conn.acknowledge_received_data() when data has been received.
• Pass the correct data size: event.flow_controlled_length from the DataReceived event to this function.