code: protected void specifyContentType(RouteContext routeContext) {
          if (!declaredProduces.isEmpty()) {
              // Specify first Produces content-type
              String defaultContentType = declaredProduces.get(0);
              routeContext.getResponse().contentType(defaultContentType);
  
              if (declaredProduces.size() > 1) {
                  // negotiate content-type from Request Accept/Content-Type
                  routeContext.negotiateContentType();
              }
          }
      }
docs: Specify the Response content-type by...
  <ol>
  <li>setting the first Produces content type</li>
  <li>negotiating with the Request if multiple content-types are specified in Produces</li>
  </ol>
  
  @param routeContext