Multipart Form content-type

[rucciva]

Hello, thanks a lot for making this library.

I have question regaring multipart support.
Does the generated code support setting the part content-type?

it does seems to handle the encoding correctly when encoding the data but it doesn't seem to add the encoding as the header

this is the generated code when using `"encoding" : {"metadata": {"contentType": "application/json"}}

    const formData = new FormData();
    if (artifactDraftUpdate.metadata !== undefined) {
      formData.append(`metadata`, JSON.stringify(artifactDraftUpdate.metadata));
    }
    if (artifactDraftUpdate.data !== undefined) {
      formData.append(`data`, artifactDraftUpdate.data);
    }