[WIP] [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 | 
| License       | MIT
| Doc PR        | 

This will output the actual value of a DateTime object in the form panel of the web debug toolbar.

- [ ] gather feedback for my change
- [ ] submit changes to the documentation (if needed)
- [ ] finish the code
can you do ``if ($value instanceof \DateTime || $value instanceof \DateTimeInterface) {`` to support the immutable datetimes of PHP 5.5 ?