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've modified the `setAttribute` method of `Node` so it doesn't decode attributes any further, as this seemed like the cleanest solution.