Allow extraHeaders to be set for browser clients in XHR requests

[darrachequesne]

To be used as (after #518 is merged):

io({ 
  transportOptions: {
    polling: {
      extraHeaders: { 'X-Requested-With': 'XMLHttpRequest' }
    },
  }
});

Closes #410

[lu4]

@darrachequesne The feature doesn't seem to be working (according to examples from Readme.md)... The extraHeaders are omitted...

Browser code:

let engine = engineio({
    transportOptions: {
        polling: {
            extraHeaders: {
                'X-TEST': 'ZZZ'
            }
        }
    }
});

engine.on('open', () => {
    console.log(['open', engine]);

    engine.send('another test');

    engine.on('message', (data) => {
        console.log(['message', data]);
    });

    engine.on('close', () => {
        console.log(['close', engine]);
    });
});

Server code:

let engineServer = engine.attach(httpsServer);

engineServer.on('connection', function (socket: any) {
    debugger; // socket.request.headers does not contain X-TEST header
    console.log(['connection', socket]);

    socket.send('test');

    socket.on('message', function(data: any) {
        console.log(['message', data, socket]);
    });

    socket.on('close', function() {
        console.log(['close', socket]);
    });
});

The problem seems to be little bit further along the road, to be specific with

engine.io-client/lib/transports/polling-xhr.js

Line 200 in b1baf8d

xhr.setDisableHeaderCheck(true);

in xhr.setDisableHeaderCheck(true); xhr doesn't contain setDisableHeaderCheck, so what's the reason to accept extraHeaders if they will be cancelled later?

[mikermcneil]

also related to #554