> Values are formatted as with an encoding/json.Encoder with SetEscapeHTML(false), with two exceptions.

> First, an Attr whose Value is of type error is formatted as a string, by calling its Error method. Only errors in Attrs receive this special treatment, not errors embedded in structs, slices, maps or other data structures that are processed by the encoding/json package.

So the json handler more or less works as if you called json.Marshal, which sounds pretty reasonable.