Using dtoa of mode=0, we can determine the number of digits in decimal that is
necessary to represent the given Float number without errors.
This change permits digits=0 in BigDecimal(flt) and Float#to_d, and these
methods use dtoa of mode=0 when the given digits is 0.
Internal implicit conversion from Float also uses digits=0.
[Fix GH-70]
https://github.com/ruby/bigdecimal/commit/2dbe170e35