fixed the format of the request used to render an exception
Sets _format attribute only if it wasn't set previously by the user.

Fixes #8787
fixed request format of sub-requests when explicitely set by the developer (closes #8787)
merged branch fabpot/inline-fragment-format (PR #8821)

This PR was merged into the 2.2 branch.

Discussion
----------

Sets _format attribute only if it wasn't set previously by the user

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8787, #8791
| License       | MIT
| Doc PR        |

Commits
-------

9bb7a3d fixed request format of sub-requests when explicitely set by the developer (closes #8787)
fa35597 Sets _format attribute only if it wasn't set previously by the user.
Merge branch '2.2' into 2.3

* 2.2:
  [Locale] fixed build-data exit code in case of an error
  fixed request format of sub-requests when explicitely set by the developer (closes #8787)
  Sets _format attribute only if it wasn't set previously by the user.
  Exclude little words of 'ee' to 'oo' plural transformation
  fixed the format of the request used to render an exception
  Fix typo in the check_path validator
  added a missing use statement (closes #8808)
  fix for Process:isSuccessful()

Conflicts:
	UPGRADE-3.0.md
	src/Symfony/Component/Locale/Resources/data/build-data.php