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
| 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.

The variable was probably named $notModified to follow the related HTTP status code name (304 NOT MODIFIED), I think you should keep the negation