public String getCharacterEncoding() {
        return DEFAULT_CHARACTER_ENCODING;
    }
The name of the encoding for the response contents, if applicable. Return
null if character set is not applicable to the response in question (e.g.
binary formats). If null is returned, not "charset" element is added to
the content type header.

@return by default {@link HttpResponse#DEFAULT_CHARACTER_ENCODING}
container-core/src/main/java/com/yahoo/container/jdisc/HttpResponse.java