Fix Event.Mock for core 1.4 nocompat

https://github.com/mootools/mootools-core/blob/ceca0cd9c3a6879c42894741134b780439bfda99/Source/Types/DOMEvent.js#L116-119
Merge pull request #17 from megawac/patch-1

Fix Event.Mock for core 1.4 nocompat