[HttpKernel] added value of DateTime object to ValueExporter

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

This will output the actual value of a DateTime object in the form panel of the web debug toolbar.

- [x] gather feedback for my change (thank you stof, its my first contribution to oss)
- [x] finish the code
I would rather use ``class_exists('DateTimeImmutable', false)``, so that the test works fine for HHVM (it implements DateTimeImmutable but it reports inconsistent PHP version in the different available ways to get it: https://github.com/facebook/hhvm/issues/2934)
Detecting the feature we need is safer here