Simplifies iteration over container's contents.
Accepts the @previousKey, returns the next key.
If @previousKey == @endKey the function returns the first key.
The function always returns so-called 'valid' keys (the ones != @endKey).
The function returns @endKey ('invalid' key) only once to signal that iteration has reached its end.
In most cases, if the map doesn't contain an invalid key ("" for JMap, None form-key for JFormMap)
it's ok to omit the @endKey.

Usage:

  string key = JMap.nextKey(map, previousKey="", endKey="")
  while key != ""
    <retrieve values here>
    key = JMap.nextKey(map, key, endKey="")
  endwhile