code: public Result addToFlash(String key, String value) {
          current(true).flash().put(key, value);
          return this;
      }
docs: Adds the given key-value pair to the current flash. This method requires a current HTTP context. If none, a
  {@link java.lang.IllegalStateException} is thrown.
  
  @param key   the key
  @param value the value
  @return the current result