LagartoDOMBuilder double-decodes attribute values

If an XML attribute contains &-encoded data (which is therefore double-encoded for storage in an attribute), it is double-decoded - once in `LagartoParser`, and again in the constructor for `Attribute` - when it is parsed by `LagartoDOMBuilder`. I suspect the `Attribute` constructor is incorrect, since this also means that `Attribute.clone` would decode the attribute value further.