net.websocket: fix incorrect formatting for custom headers during handshake

[zUltimaPG3D]

fix that makes custom headers during ws client handshake actually valid

if for example a server requires the Origin header to be set, before this fix, said server would've denied the connection even if the value was correct and the header was there, just because the header isn't valid, but now since it's valid it no longer fails

i didn't make a specific test for this fix in particular because it just adds a space to a string and the only valid testing option i know of is a very specific domain (not local) and i wouldn't want to have an external connection in a test like that

[huly-for-github]

Connected to Huly®: V_0.6-26084

[spytheman]

Afaik .github/workflows/websockets_ci.yml already does more extensive tests, by starting a docker container with an Autobahn server, and running our client against it.

Can the vlib/net/websocket/tests/autobahn/autobahn_client.v program be changed to add at least one custom header?

That will change the handshake request, and hopefully can serve as a regression test, without needing to add a separate test that does a websocket request to a separate external sertice.