qbs_str my mod

[Jack]

I have an idea, you could set the max-digits to 16 and make two conversions to string, one with 15 digits and the other with 16 digits, if the exponents are the same then use 16 digits else use 15
it's just a matter of doing it, but I think I have spring laziness 
on the other hand restricting double to string conversion to 15 digits you are less likely to have output like 4.99999999999999 so I vote to leave it at 15