add checkServerIdentity option

[djgorilla]

Note: the engine.io.js file is the generated output of make engine.io.js, and should not be manually modified.

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

not able to set checkServerIdentity

New behaviour

able to set checkServerIdentity

Other information (e.g. related issues)

const io = require('socket.io-client');
const fs = require('fs')

const client = io('wss://localhost:3030', {
transports: ['websocket'],
secure: true,
ca: fs.readFileSync('./server_cert.pem'),
cert: fs.readFileSync('./user_cert.pem'),
key: fs.readFileSync('./user_key.pem'),
checkServerIdentity: () => { return null },
});

[baileyherbert]

@darrachequesne Please check on this simple addition! Until it's merged, engine.io-client will not work for self-signed, hostless SSL certificates, which are common for internal networking – https://nodejs.org/api/tls.html#tls_tls_checkserveridentity_hostname_cert

[darrachequesne]

Merged as d134fee. Thanks!