Fix lost error in Reduce for single-element streams

[destel]

When the input stream contained exactly one element and it was an error, Reduce returned nil, true instead of propagating the error.
Added a test for single-element streams and fixed the bug.
Fixes #71

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.03%. Comparing base (d1f334c) to head (ca40771).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
- Coverage   99.11%   99.03%   -0.08%     
==========================================
  Files          17       17              
  Lines        1016      832     -184     
==========================================
- Hits         1007      824     -183     
+ Misses          9        8       -1     

☔ 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.