Fix rounding with decimal128_t

[mborland]

Closes: #1029

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.3%. Comparing base (51d8181) to head (801ff24).
⚠️ Report is 19 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1039     +/-   ##
=========================================
+ Coverage     98.3%   98.3%   +0.1%     
=========================================
  Files          249     249             
  Lines        16485   16491      +6     
  Branches      1851    1855      +4     
=========================================
+ Hits         16197   16209     +12     
+ Misses         288     282      -6     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	97.9% <100.0%> (+1.6%)	⬆️
test/github_issue_1026.cpp	100.0% <100.0%> (ø)
test/test_constants.cpp	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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