JsonBindingProvider use incorrect charset encoding

With a rest resource method returning a POJO and annotated with @Produces(Mediatype.APPLICATION_JSON), the returned response response is incorrectly encoded (uses US_ASCII instead of utf-8).

The workaround is to add a parameter to the mediatype: @Produces("application/json;charset=utf-8").

There is 2 issues concerning compliance with the RFC7159 json spec:

• As per the json spec, charset parameters should be discarded:

  Note: No "charset" parameter is defined for this registration. Adding one really has no effect on compliant recipients.

• As per the json spec, json is utf-8 encoded:

  JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default encoding is UTF-8, and JSON texts that are encoded in UTF-8 are interoperable in the sense that they will be read successfully by the maximum number of implementations

The source of these issues lies in org.jboss.resteasy.plugins.providers.jsonb.AbstractJsonBindingProvider#getCharset, which attempts to figure out the charset to use, defaulting to Charset.defaultCharset(), whereas utf-8 should be used and this logic is not required:

 public static Charset getCharset(final MediaType mediaType) {
      if (mediaType != null)
      {
         String charset = mediaType.getParameters().get("charset");
         if (charset != null) return Charset.forName(charset);
      }
      return Charset.defaultCharset();
   }

This method is called from org.jboss.resteasy.plugins.providers.jsonb.JsonBindingProvider#writeTo:

entityStream.write(jsonb.toJson(t).getBytes(getCharset(mediaType)));