Send the client-side JSON in UTF-8 #149

The client-side JSON endpoint (/51dpipeline/json) sends application/json written through the response's writer in the response's character encoding (ClientsidePropertyServiceCore.java line 281 at 30e48867). The servlet API makes that ISO-8859-1 when the application sets none, so a value with a character outside Latin-1, such as a city name, reaches the browser as bytes that are not UTF-8.

RFC 8259 section 8.1 says "JSON text exchanged between systems that are not part of a closed ecosystem MUST be encoded using UTF-8", and section 11 defines no charset parameter.

Write the JSON's UTF-8 bytes through the output stream with Content-Length counted in those bytes, and leave Content-Type as application/json, as the specification's table gives it. The JavaScript endpoint is written the same way, so decide its encoding in the same change, and whether its Content-Type should name the charset.

Test with a value outside Latin-1 and no encoding set by the application, red first.

Found in the review of 51degrees-java#67.