Fix upgrade check

The checks in the ```/socket``` handler were invalid and caused preconditions to fail when using a browser to access that path.

```
Mai 07 16:02:06 stef.thewalter.lan cockpit-ws[8239]: g_ascii_strcasecmp: assertion 's1 != NULL' failed
Mai 07 16:02:06 stef.thewalter.lan cockpit-ws[8239]: received invalid or missing Upgrade header: (null)
Mai 07 16:02:06 stef.thewalter.lan cockpit-ws[8239]: Received invalid handshake request from the client
Mai 07 16:02:06 stef.thewalter.lan cockpit-ws[8239]: Connection from (null):0 for stef closed
```