[HttpFoundation] Fix PdoSessionHandler to work properly with streams
minor #12146 [HttpFoundation] Fix PdoSessionHandler to work properly with streams (rybakit)

This PR was squashed before being merged into the 2.6-dev branch (closes #12146).

Discussion
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[HttpFoundation] Fix PdoSessionHandler to work properly with streams

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Ref: https://github.com/symfony/symfony/pull/10931#discussion_r18387931

Commits
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9531a2b [HttpFoundation] Fix PdoSessionHandler to work properly with streams