Both generic/2/2 and generic/2/3 do not send WINDOW_UPDATE on connection

[velavokr]

They only send a stream-level WINDOW_UPDATE and then expect to receive a DATA frame.

The problem is that a well-behaving http/2 server is not allowed to send non-empty DATA unless the connection-level flow is also unblocked.

[velavokr]

Two flow-control windows are applicable: the stream flow-control window and the connection flow-control window. The sender MUST NOT send a flow-controlled frame with a length that exceeds the space available in either of the flow-control windows advertised by the receiver. Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow-control window.