- Used by the Angular XHRBackend service.
- Only supporting 'load' and 'error' now.
- Not implemented as weak events. Client responsible for detaching or not
keeping a reference to the XHR object forever. (NG XHRBackend does so)
In order to follow the spec the XHR statusText property should
return just the status text without the status code. Adding
tests to guard this behavior.