HTTP/2: flushing of the SSL buffer in transition to the idle state.

HTTP/2: flushing of the SSL buffer in transition to the idle state.

It fixes potential connection leak if some unsent data was left in the SSL
buffer.  Particularly, that could happen when a client canceled the stream
after the HEADERS frame has already been created.  In this case no other
frames might be produced and the HEADERS frame alone didn't flush the buffer.