code: protected <T> T getValueFromJson(String json, String propName) throws IOException {
          return (T) jsonStringToMap(json).get(propName);
      }
docs: A helper method that takes a json string and extracts the value of the specified
  property.
  
  @param json     the json string
  @param propName the name of the property to extract
  @param <T>      The type for the value expected to be returned.
  @return the value of the specified property
  @throws IOException