moved the request scope creation to the ContainerAwareHttpKernel class
[DependencyInjection] fixed PHP notice when the scope is not defined
merged branch fabpot/request-scope (PR #7457)

This PR was merged into the master branch.

Discussion
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moved the request scope creation to the ContainerAwareHttpKernel class

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#2343

While updating the scope documentation, I realized that the request scope was created in the FrameworkBundle while the HttpKernel that manages it was in the HttpKernel component. So, this PR makes things more consistent.

Commits
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cec98c1 [DependencyInjection] fixed PHP notice when the scope is not defined
550df5a moved the request scope creation to the ContainerAwareHttpKernel class