same tweets are fetched or inifinite loop occurs in doRppAPICall

doRppAPICall fetches same tweets if the number of tweets is less than 'num' argument,
because max_id_str is used as max_id parameter.
max_id_str should be replaced with since_id_str.

In addition, infinite loop occurs if length(newList) is 0, e.g. a future date is specified as 'since' argument.
The while loop should be terminated if length(newList) is 0.