Fix narrowing in equality of mixed types

[mborland]

Closes: #799

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (069c26a) to head (1116018).
Report is 13 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #810     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          227     228      +1     
  Lines        16967   16978     +11     
  Branches      1809    1809             
=========================================
+ Hits         16775   16786     +11     
  Misses         192     192             

Files with missing lines	Coverage Δ
include/boost/decimal/detail/comparison.hpp	99.4% <ø> (ø)
test/github_issue_799.cpp	100.0% <100.0%> (ø)

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