Response::isNotModified returns true when If-Modified-Since is later than Last-Modified
bug #11079 Response::isNotModified returns true when If-Modified-Since is later than Last-Modified (skolodyazhnyy)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #11079).

Discussion
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Response::isNotModified returns true when If-Modified-Since is later than Last-Modified

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10501,  #10857
| License       | MIT
| Doc PR        |

Patch for #10501. I have reworked a bit `Response::isNotModified` method to make it more readable. Now it's comparing If-Modified-Since and Last-Modified as dates.

Commits
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42ec76e Response::isNotModified returns true when If-Modified-Since is later than Last-Modified