Fix default throughput monitoring

[graebm]

The last PR didn't actually work: #263
That's what happens when you don't add tests

Thank you @zoewangg for pointing out minimum bytes should be 1, not 0.
Thank you @TingDaoK for the awesome mock server that let us emulate crazy server behavior.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #265 (6662a80) into main (9566422) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
- Coverage   87.17%   87.06%   -0.11%     
==========================================
  Files          16       16              
  Lines        4046     4036      -10     
==========================================
- Hits         3527     3514      -13     
- Misses        519      522       +3     

Impacted Files	Coverage Δ
source/s3_client.c	86.65% <100.00%> (-0.02%)	⬇️

... and 2 files with indirect coverage changes