Send additional cookies

[jaykravetz]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behaviour

During the handshake, only the io cookie can be sent

New behaviour

This PR allows the override of a function (getAdditionalCookies) to send any other cookies required

Other information (e.g. related issues)

See issue #557

Any and all feedback is greatly appreciated.
Keep up the good work!

[darrachequesne]

Superseded by 2527543, included in [email protected].

Syntax:

server.on("initial_headers", (headers, req) => {
  headers["test"] = "123";
  headers["set-cookie"] = "mycookie=456";
});

server.on("headers", (headers, req) => {
  headers["test"] = "789";
});

Thanks 👍