Update observable.js

应该在if语句中补充“ ret=false; ”，否则在下面例子中，不能阻止表单的默认提交。
测试代码（可以在控制台尝试此代码）：
var S = KISSY, D = S.DOM, E = S.Event;
var f = S.get('#J_Form');
function fireSubmitEvent(el) {
    var canSubmit = false;
    canSubmit  = E.fire(el, 'submit');
    return canSubmit;
}
var bak = f.submit;
f.submit = function(){
    var canSubmit = fireSubmitEvent(this);
    console.log('canSubmit:'+canSubmit); alert(canSubmit);
    canSubmit && bak.call(this);
}
//E.on(f, 'submit', function(e){ console.log('can on!'); alert(2); return false; });
f.onsubmit = function(){ alert(1); return false; };
f.submit();