TELNET control and environment data bypass HTTPS-proxy TLS

I found a way to make libcurl send TELNET credential and environment data in cleartext to a TELNET origin, even when the connection is supposedly protected by an HTTPS proxy tunnel. The tunnel's TLS never sees it.

// lib/telnet.c:1044-1047
bytes_written = swrite(conn->sock[FIRSTSOCKET], temp, len);
if(bytes_written < 0)
  failf(data, "Sending data failed (%d)", SOCKERRNO);