multi-byte characters are altered or lost when encoding as JSON

(This issue originates from a [discuss post](https://discuss.elastic.co/t/filebeat-disk-queue-messing-up-encoding/260546) about the Beats disk queue)

UTF8-encoded strings are distorted when encoding and then decoding structs to JSON, e.g.:

```json
{"modUserFullName":"Name With Polish Chars łóąść"}
```

This arises in the Beats disk queue since in the initial version `go-structform` is used to serialize to and from JSON, which corrupted events with UTF8 strings.