Fix all 9s rounding on the right side of the decimal point with fixed precision

[mborland]

Closes: #267

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.86%. Comparing base (38854b2) to head (ba6a62d).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #269      +/-   ##
===========================================
+ Coverage    94.85%   94.86%   +0.01%     
===========================================
  Files           69       70       +1     
  Lines         9077     9097      +20     
===========================================
+ Hits          8610     8630      +20     
  Misses         467      467              

Files with missing lines	Coverage Δ
include/boost/charconv/detail/dragonbox/floff.hpp	87.10% <100.00%> (ø)
test/github_issue_267.cpp	100.00% <100.00%> (ø)

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