JSON responses are parsed to text as iso-8859-1 by default rather than UTF-8

[svella]

Per https://tools.ietf.org/html/rfc7159#section-8.1

JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default encoding is UTF-8 ...

but in the absence of an explicit charset specified in the content-type header, ResponseBuilder.build parses it into text using iso-8859-1 (via HttpResponse.getContentAsString()) which isn't even a legal charset for JSON.