introduce support for http-streaming by not filtering multiple LOADING e...

...vents following the first one.

Hi,

I noticed that multiple LOADING events were filtered out instead of being forwarded to listeners of onreadystatechange. 
I'm not actually sure the multiple "LOADING" event is part of the W3C specs but you can see it working on most browsers (at least Firefox Chrome Safari and IE starting from IE10)

e.g.: try this:
```JavaScript
var XHRClass = typeof XMLHttpRequest != "undefined" ? XMLHttpRequest : require("xmlhttprequest").XMLHttpRequest;  
var xhr = new XHRClass();
xhr.onreadystatechange = function() {
  console.log("State: " + this.readyState);
};
xhr.open("GET", "http://push.lightstreamer.com/lightstreamer/create_session.js?LS_client_version=5.0&LS_adapter_set=DEMO&LS_keepalive_millis=1000");
xhr.send();
```

My pull request contains the very small fix and a test derived from test-events (I should have probably stripped more stuff from the original test, let me know if you want me to)

let me know your thoughts,
Bye.