code: public JWTPayload setSubject(String subject) {
          this.subject = subject;
          this.put(PayloadConstants.SUBJECT, subject);
          return this;
      }
docs: Sets the subject claim identifying the principal that is the subject of the JWT or {@code null} for none.
  
  <p>
  Overriding is only supported for the purpose of calling the super implementation and changing
  the return type, but nothing else.
  </p>