fix overwriting of request's locale if attribute _locale is missing
merged branch uwej711/fix_locale_is_overridden_2_1 (PR #7686)

This PR was merged into the 2.1 branch.

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

In addition to https://github.com/symfony/symfony/pull/7516 same change for the 2.1 branch, see comments on https://github.com/lunetics/LocaleBundle/pull/59

Commits
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da156d3 fix overwriting of request's locale if attribute _locale is missing