function XHR (opts) {
	  Polling.call(this, opts);
	  this.requestTimeout = opts.requestTimeout;
	  this.extraHeaders = opts.extraHeaders;
	
	  if (typeof location !== 'undefined') {
	    var isSSL = 'https:' === location.protocol;
	    var port = location.port;
	
	    // some user agents have empty `location.port`
	    if (!port) {
	      port = isSSL ? 443 : 80;
	    }
	
	    this.xd = (typeof location !== 'undefined' && opts.hostname !== location.hostname) ||
	      port !== opts.port;
	    this.xs = opts.secure !== isSSL;
	  }
	}
XHR Polling constructor.

@param {Object} opts
@api public