check enablesXDR option when Request.hasXDR is called just in case

Add a fix to primus/primus#282 together with https://github.com/Automattic/engine.io-client/pull/333.
It should work without this fix because IE8 uses jsonp-polling when enablesXDR is false, but this way makes more sense, and possibly avoid bugs in future.
This should be included in the browser only section (under `if (env.browser)`)