code: public JWTHeader setContentType(String contentType) {
          this.contentType = contentType;
          this.put(HeaderConstants.CONTENT_TYPE, contentType);
          return this;
      }
docs: Sets the content type header parameter used to declare structural information about 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>