fix overwriting of request's locale if attribute _locale is missing
merged branch uwej711/fix_locale_is_overridden (PR #7516)

This PR was merged into the 2.2 branch.

Discussion
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[HttpKernel] fix overwriting of request's locale by LocaleListener if attribute _locale is missing

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7474
| License       | MIT
| Doc PR        | n.a.

Commits
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e8cae94 fix overwriting of request's locale if attribute _locale is missing