fixed locale of sub-requests when explicitely set by the developer (refs #8821)
Fixed issue #6932 - Inconsistent locale handling in subrequests

The fix consists in passing the locale in the controller reference,
inside the routablefragment renderer.
[HttpKernel] added a comment to warn about possible inconsistencies
fixed request format when forwarding a request
merged branch fabpot/format-forward (PR #8829)

This PR was merged into the 2.2 branch.

Discussion
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fixed request format when forwarding a request

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6932
| License       | MIT
| Doc PR        | n/a

When calling forward() in a controller, the format is not the same as for the master request.

Commits
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7e87eb1 fixed request format when forwarding a request