getFeatureFlagPayload returns non strigified JSON

[marandaneto]

💡 Motivation and Context

Relates to PostHog/posthog-ios#49

💚 How did you test it?

Unit tests and creating a feature flag per type and testing them manually.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[benjackwhite]

Very nice 👍

[neilkakkar]

thank you!