code: public static <K, V> Function<Map.Entry<K, V>, String> toStringWithKeyValueSeparator(
        final String separator) {
      return new EntryJoinerFunction<>(separator);
    }
docs: Returns a Guava {@link Function} to transform map entries to strings by taking the {@link
  Object#toString()} of the key and the value and joining them with the supplied {@code
  separator}. None of the key, value, or separator may be null.