code: @Override
      public Resource createRelative(String relativePath) {
          String pathToUse = StringUtils.applyRelativePath(this.path, relativePath);
          return new ClassPathResource(pathToUse, this.classLoader, this.clazz);
      }
docs: This implementation creates a ClassPathResource, applying the given path
  relative to the path of the underlying resource of this descriptor.
  
  @return the resource.
  @see spark.utils.StringUtils#applyRelativePath(String, String)