fix: respect last-modified from mapped metadata

[tripodsan]

fixes #737

backporting #738

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (379796e) to head (6bbf7d9). Report is 11 commits behind head on 5.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 5.x #742 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 50 50 Lines 4480 4514 +34 ========================================= + Hits 4480 4514 +34 ```

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[github-actions[bot]]

This PR will trigger a patch release when merged.

[tripodsan]

won't fix.