[go] Serialize multipart array of complex objects as JSON

Created by: thiagoarrais

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cc @antihax @bvwells @grokify @kemokemo

Description of the PR

Fixes #2964 (closed) by forcing multipart items that are complex objects to be rendered as json.

I'm not really sure this is the correct approach architecturally. Just let me know if you need any changes. And please also help me see the many hidden edge cases I'm not finding.

Update: Fixes #3043 (closed)