[BUG] fix error response in case of server error

[BUG] fix error response in case of server error

The fix below was incomplete :
    commit d5fd51c75be6479539228f84377622a986b23be2

    [BUG] http_server_error() must not purge a previous pending response

    This can cause parts of responses to be truncated in case of
    pipelined requests if the second request generates an error
    before the first request is completely flushed.

Pending response data being rejected was still sent, causing inappropriate
error responses in case of error while parsing a response header. We must
purge pending data from the response buffer that were not scheduled to be
sent (l - send_max).