Sending an empty “Server” header during handshake
will make Safari error out with

    “WebSocket connection [...] failed: Invalid UTF-8
    sequence in header value”

This is an open bug in WebKit:
https://bugs.webkit.org/show_bug.cgi?id=139298



As far as I see, “Server” is not a required handshake
header as specified in RFC 6455 section 4.2.2 (5),
and we can omit it if the server name is not set.

Change-Id: Icec142f867813c16e3e3baa1378582164242e049
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>