Fix for #535 ExtraHeaders doesn't work in browser

[lu4]

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

xhr.setDisableHeaderCheck is not introduced in some browsers so engine.io fails to set extraHeaders which makes feature #519 to fail.

New behaviour

xhr.setDisableHeaderCheck is checked to exist before call, in case if any secure headers are modified, browser throws error to prevent fraud.

Other information (e.g. related issues)

Please provide feedback in case if any PR rules are violated

[darrachequesne]

Great catch, thanks! I've just reverted the change to the output file engine.io.js, as it is automatically generated.

[lu4]

Thanks, yeah I was unsure if that was necessary move, so created a separate commit,

Regards,
Denis

[darrachequesne]

@lu4 out of curiosity, in your example here:

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

Won't the X-TEST header generate an OPTION request, which will result in a 400 response from the engine.io server? (I'm wondering if there is something to fix on our side, ref: socketio/engine.io#279)

[lu4]

It wasn't my case, everything were working smoothly, no option request was generated... I didn't mean to use any predefined headers, just a first thing that came of my head...