[HttpKernel] added value of DateTime object and objects implementing DateTimeInterface to ValueExporter
feature #11070 [HttpKernel] added value of DateTime object to ValueExporter (scuben)

This PR was merged into the 2.6-dev branch.

Discussion
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[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

Commits
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a1762fb [HttpKernel] added value of DateTime object and objects implementing DateTimeInterface to ValueExporter