Preserve OpenAPI request content type and encode bodies accordingly

[ncoop57]

Problem

fastspec currently sends request bodies as JSON for generated OpenAPI calls. This works for JSON-first APIs, but not for Stripe v1 write endpoints, which expect application/x-www-form-urlencoded.

Stripe v1 also expects nested structures to be flattened:

• metadata={"kind": "subscription"} -> metadata[kind]=subscription
• items=[{"price": "price_..."}] -> items[0][price]=price_...