Cast button value as string in postprocess (#7733)
* Cast button value as string.
If you don't do this, it causes a problem on gradio app load - see github issue #7690
(https://github.com/gradio-app/gradio/issues/7690#issuecomment-1997718185)
* add changeset
* add test
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Co-authored-by: gradio-pr-bot <gradio-pr-bot@users.noreply.github.com>
Co-authored-by: Abubakar Abid <abubakar@huggingface.co>
