Fallback to default attribute values when invalid values are used.

When an `ATTRS` setter returns `Y.Attribute.INVALID_VALUE` during initialization, and a default attribute value is defined, fallback to that value.

Fixes [#2528732](http://yuilibrary.com/projects/yui3/ticket/2528732)
Trumps https://github.com/yui/yui3/pull/48