Fix always firing onConfigChanged hook

[z4kn4fein]

Describe the purpose of your pull request

This PR fixes an issue that the configChanged hook is emitted in browser environments even when the downloaded config JSON isn't changed.
Part of the issue is that we get 200 response from the browser even when the server responds with 304 due to the Cache-Control: max-age=0, must revalidate header we introduced to bypass unnecessary OPTIONS requests.

Related issues (only if applicable)

JS-SDK issue

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (406d749) 92.24% compared to head (7250320) 92.24%.

❗ Current head 7250320 differs from pull request most recent head 896b2d6. Consider uploading reports for the commit 896b2d6 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files          18       18           
  Lines        1496     1496           
  Branches      380      380           
=======================================
  Hits         1380     1380           
  Misses        115      115           
  Partials        1        1           

Impacted Files	Coverage Δ
src/ConfigServiceBase.ts	88.43% <100.00%> (ø)

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication