Bugfix unblock sending when the connection closes

This allows the send_task to unblock and exit when the connection
closes. This should prevent these warnings,

    Task was destroyed but it is pending!
    task: <Task pending coro=<ProtocolWrapper.send_task() done, defined at hypercorn/protocol/__init__.py:58> wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x7f08fc3db1d0>()]>>

src/hypercorn/protocol/h2.py

@@ -346,3 +346,4 @@ async def _close_stream(self, stream_id: int) -> None:
         if stream_id in self.streams:
             stream = self.streams.pop(stream_id)
             await stream.handle(StreamClosed(stream_id=stream_id))
+            await self.has_data.set()