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

[Form] [Extension] [ChoiceType] This fixes issue #10409 by not using json_encode() anymore, but serialize() instead. See issue for more information or this forum post I found: 
http://forum.symfony-project.org/viewtopic.php?f=23&t=70133

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10409
| License       | MIT
| Doc PR        | N/A