[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
This should not be in CHANGELOG-2.5 which is an auto-generated file (you should not change it). It should be in the CHANGELOG file of the HttpKernel component