fix(xhr): add cache busting for safari, webview android and ios

Because of a ridiculous cache for cross-origin ressources in webkit (cf. https://bugs.webkit.org/show_bug.cgi?id=82672) we need to ensure that we never ask for cached data in those devices.

This must fix this issue (https://github.com/Polymer/ShadowDOM/issues/516) in the ShadowDOM repository.

Maybe we should enable this only when we load cross-origin ressources to have a better cache control.