bug symfony#18243 Fixed a bug: saving with not serializable values (RedisAdapter) (masterklavi)

This PR was squashed before being merged into the 3.1-dev branch (closes symfony#18243).

Discussion
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Fixed a bug: saving with not serializable values (RedisAdapter)

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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d86173c Fixed a bug: saving with not serializable values (RedisAdapter)

Author: nicolas-grekas