corrected function optional_t

Removed the line ' default = optional_s(default, nil) ' which is not doing anything (default always keeps its current value after this line, irrespective of being nil or not). 
Corrected the second case ' elseif default ~= nil ... ' which would previously behave wrongly for boolean default value 'false'.
Merge pull request #11 from yulangie/patch-1

corrected function optional_t