Fix up rounding for small values.

[dereuromark]

Fix up issues found in spryker/decimal-object#25

Thanks to AI I got a few leads on what the issues are.
With more test coverage this should have fixed the issues.

Let me know if there are any cases left.
@gahorth

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.16%. Comparing base (2f95de3) to head (eeb19aa).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #18      +/-   ##
============================================
+ Coverage     93.96%   94.16%   +0.20%     
- Complexity      101      103       +2     
============================================
  Files             1        1              
  Lines           232      240       +8     
============================================
+ Hits            218      226       +8     
  Misses           14       14              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.