Fix incorrect RoundUp conversion of large strings

[jmkuhn]

Closes #176

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.14%. Comparing base (cf9aef3) to head (2fa551b).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
+ Coverage   95.81%   96.14%   +0.33%     
==========================================
  Files           2        2              
  Lines         597      597              
==========================================
+ Hits          572      574       +2     
+ Misses         25       23       -2     

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[stevengj]

LGTM. Merge and tag a new release?