[must-revalidate] properly revalidate based on eTag

[edwinveldhuizen]

Fixes #822 and by fixing #507 in a different fashion

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.26%. Comparing base (e9d05ed) to head (039da87).
Report is 9 commits behind head on main.

❗ Current head 039da87 differs from pull request most recent head 2916549. Consider uploading reports for the commit 2916549 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #824      +/-   ##
==========================================
- Coverage   99.29%   99.26%   -0.03%     
==========================================
  Files          19       19              
  Lines        2400     2449      +49     
  Branches      211      214       +3     
==========================================
+ Hits         2383     2431      +48     
- Misses         16       17       +1     
  Partials        1        1              

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[arthurfiorette]

thanks!