[Bug] Blaze: hidden html attribute does not work properly

Blaze dynamic attributes doesn't work properly with attribute **hidden**.

If you use:

```
helperName(){
    return {
        checked: false
}
```

It works great, removing the checked attribute.
But when you use:

```
helperName(){
    return {
        hidden: false
}
```

Blaze creates a html with:

```
hidden="false"
```

that occurs in an hidden element.
