The issue is that this buffer is also emptied at the end.

Current version assumes that the server thread will not have enough opportunity to send the whole buffer before receiving a RST.

But in reality, it's quite possible (happens reliably on a box) that:
1. many server sent chars get merged in one packet
and/or
2. RST is yielded only once the server have put all its buffer in send queue.

As a consequence, server's buffer is empty at the time it is checked, and the test is marked as failed although everything worked fine.