headers with JSON values are incorrectly handled in FFI

[mefellows]

Upstream issue for pact-foundation/pact-js#1058

Repro here: https://github.com/pact-foundation/pact-reference/compare/repro/pact-js-issue-1058.

Note the rust consumer test works as expected, so it's somewhere in the FFI layer.

[github-actions]

👋 Hi! The 'smartbear-supported' label has just been added to this issue, which will create an internal tracking ticket in PactFlow's Jira (PACT-1238). We will use this to prioritise and assign a team member to this task. All activity will be public on this ticket. For now, sit tight and we'll update this ticket once we have more information on the next steps.

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