feat: respect Encoding Object while building requests

[char0n]

This change fixes both:

1. making multipart/form-data requests with content-type header for every individual boundary
2. generating correct CURL command for multipart/form-data request, allowing specifying content-type header for every individual boundary

Refs #4826
Refs #5356

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multipart/form-data boundaries now containing Content-Type headers:

CURL command correctly generated by assigning additional type=:

[LaFrimousse]

Hi all.

Unfortunately, this fix does not solve #5356 .
Please see my comment.