Fix for error in Firefox because of workaround for Safari

Fix for https://github.com/Polymer/platform/issues/94
RegExp based on user agent examples from http://myip.ms/view/comp_browsers/3458/Safari_8.html beause I do not have any available machine with Safari 8
The problem with Safari (and WebKit nightlies) is that it returns `{get: undefined, set: undefined, ...}`. That is an invalid descriptor if there is no `value` property so maybe we need to ensure that there is no value as well?