Allow attributes to have a value of zero (eg min='0')

I've found that HTML rule attributes that have a value of `0` are not applied, for example, `min="0"`.  If `min="0"` is specified, the user could still enter `-1` in the `<input>` field.

This is because in `attributeRules()` there is a truthy test on the attribute's value before the rule is applied, so anything equal to `0` will be ignored.

To fix this, we also check for `value === 0`.  If so, the rule is applied.

Tests were added to validate this.