[Libvncserver-common] LibVNCServer websockets and wss:// connection with novnc

Hi Joel, Gernot,

I am currently tackling https://github.com/LibVNC/libvncserver/issues/68 and am
having a hard time setting up a wss:// connection between a LibVNCServer
example server and novnc 1.0.0. As you two are the principal authors of the
websockets code in LibVNCServer, I need your advice.

This is what I'm doing, using libvncserver master:

```
mkdir build
cd build
cmake -DWITH_GNUTLS=OFF .. # use OpenSSL as GnuTLS is simply broken, crashes
make
cd ..
cd webclients
openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes # create a self-signed cert
../build/examples/example  -sslcertfile cert.pem -sslkeyfile key.pem
```

and open http://localhost:5800/novnc/vnc.html?autoconnect=true&host=localhost&port=5900&encrypt=true in
a browser. However, I get

```
11/11/2018 17:22:37 Got connection from client 127.0.0.1
11/11/2018 17:22:37   other clients:
11/11/2018 17:22:37 Got TLS/SSL WebSockets connection
11/11/2018 17:22:37 webSocketsHandshake: invalid client header
11/11/2018 17:22:37 Client 127.0.0.1 gone
```

Am I doing something wrong? 

If it worked with former versions - any idea where it could have been broken?

Thanks for your thoughts,

   Christian