[HttpKernel] fixed locale management when exiting sub-requests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7063
| License       | MIT
| Doc PR        | n/a

This fix is temporary as #7007 will fix it properly in Symfony 2.3.