Fixed bug in ChoiceType triggering a warning when not using utf-8

This fixes issue #10409 by not using json_encode anymore, but serialize
instead.
Added test case for 4c6a2d15095c13b2a35751b2b2712b183be489c4
bug #10568 [Form] Fixed hashing of choice lists containing non-UTF-8 characters (webmozart)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Fixed hashing of choice lists containing non-UTF-8 characters

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10409
| License       | MIT
| Doc PR        | -

Commits
-------

18dc9a7 Added test case for 4c6a2d15095c13b2a35751b2b2712b183be489c4
73d56f7 Fixed bug in ChoiceType triggering a warning when not using utf-8