Properly handle nested params

[ioquatix]

https://github.com/socketry/multipart-post/pull/52

Types of Changes

• Bug fix.
• New feature.

Testing

• [ ] I added tests for my changes.
• [ ] I tested my changes locally.
• [ ] I tested my changes in staging.
• [ ] I tested my changes in production.

[ioquatix]

It turns out handling headers is a bit more complex. The part headers are indexed by key, but this won't work for nested parts, we either need:

1. Nested part headers based on keys, computed during recursion, i.e. part_headers[k] - we'd also might need to ignore non-string-key headers when constructing parts.
2. Flat part headers indexed by full key prefix/path.