[FSSDK-9486] Cherry-pick Last-Modified patch to a release

[mikechu-optimizely]

Summary

• Applying the changes from PR #355 to a release branch for patching the Last-Modified header bug

Test plan

• Unit and integration tests are expected to pass

Issues

• FSSDK-9486

[mikechu-optimizely]

356 Needs to be merged to master and then this PR's branch needs to include the changes before it can be merged.

[mikechu-optimizely]

Sorry for the excess noise from linting and the GitHub Actions workflow cherry-pick 😢

[mikechu-optimizely]

@jaeopt Whenever you have a moment between hacking, can you do a code review? Thanks! You're the best 🤩