(7.2.z) RESTEASY-2171 - 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

Checking this issue with Wireshark and this end-point:

    @GET
    @Produces("application/json")
    @Path("a")
    public CustomObject a() throws Exception {
        return new CustomObject().setA("öü");
    }

• default encoding (UTF8) + default json provider (JSON-B)
  • correct: UTF8 - "öü" - 22 c3 b6 c3 bc 22
• default encoding (UTF8) + old json provider (Jackson)
  • correct: UTF8 - "öü" - 22 c3 b6 c3 bc 22
• us-ascii + default json provider (JSON-B)
  • wrong: us-ascii - "??" - 22 3f 3f 22
  • 3f is literally '?' according to the first google response
• us-ascii + old json provider (Jackson)
  • correct: UTF8 - "öü" - 22 c3 b6 c3 bc 22

Upstream jira: https://issues.jboss.org/browse/RESTEASY-2171