fixed the format of the request used to render an exception
merged branch fabpot/exception-format (PR #8819)

This PR was merged into the 2.2 branch.

Discussion
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[HttpKernel] Fix request _format duplication for HttpKernel's ExceptionListener

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8777, #8778
| License       | MIT

This is a better fix for #8777.

Commits
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f946108 fixed the format of the request used to render an exception