Ref: https://wiki.saucelabs.com/display/DOCS/2018/10/16/Announcing+End+of+Life+for+Internet+Explorer+8+on+Windows+7

As `self` is undefined in React Native.

The commit 99bcc622 resulted in `window is not defined` errors.

[fix] Trigger xhr polling transport unload handler on 'pagehide' or 'unload' instead of 'beforeunload' (#604)

...to prevent broke connection on cancelled beforeunload event

Fixes #603 

Related: https://github.com/socketio/socket.io-client/issues/1166

React-Native provides a Websocket object that is more functionally aligned with the Node.js websocket than the browser websocket.

It has the same constructor signature as the Node.js websocket and can support extraHeaders and protocols.

This PR will detect when the engine.io-client is running in React-Native, call the proper Websocket constructor, and enable support for extraHeaders.

Excluding tests on iphone / ipad / edge for the moment.

The CI seems to fail with the new version:

/home/travis/build/[secure]io/engine.io-client/node_modules/zuul/bin/zuul:332
            throw err.message;
            ^
internet explorer@8: [eval("(window.zuul_msg_bus ? window.zuul_msg_bus.splice(0, 1000) : []);")] Not JSON response

Reference: https://nodejs.org/docs/latest-v6.x/api/tls.html#tls_tls_createsecurecontext_options

Some XHR implementations (like Firefox WebWorker, react-native and some Android 4.x versions) throw
an exception when setting xhr.responseType = 'arraybuffer' when xhr.readyState === 2 (which is
perfectly valid, spec-wise).

That commit fixes that behaviour by reverting some of the changes from
https://github.com/socketio/engine.io-client/pull/562 for those implementations.

Fixes #589
Closes #590 

Related: https://nodesecurity.io/advisories/550

Following #583.

This reverts commit 6043c49e.

Related: https://nodesecurity.io/advisories/debug_regular-expression-denial-of-service

Related: https://wiki.saucelabs.com/pages/viewpage.action?pageId=70068108

Related: https://nodesecurity.io/advisories/528.

This reverts commit be73a9c3.