Works around an issue where some browsers may support ProgressEvent without supporting the loadend event.
Provides correct workaround for browsers which support ProgressEvent but not the loadend event. Fixes bug introduced in 4f44c8fc57967c5747fa813421a1a24422438fb0 where SC.Request load (or error or abort) notifications would fail to fire in said browsers.
Adds sniffer for XHR2 features (and loadend event) to SC.platform. Updates SC.Response to use centralized SC.platform sniffers. Updates SC.Request unit tests to pass!