feat(): support multipart mixed

[alfonso-presa]

Description

This is a fix in order to support multipart mixed content-type. It's quite similar to multipart/form-data, but as Form-Data and Axios are forcing a content-type change, patching is needed.

Reported in #4178

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• The pull request only addresses one issue or adds one feature.
• The pull request does not introduce any breaking changes
• I have added screenshots or gifs to help explain the change if applicable.
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[pooja-bruno]

Hey @alfonso-presa, I reviewed the PR, and there are still one issue. The environment variables are not being interpolated in the response. If the Content-Type is multipart/mixed.

[alfonso-presa]

@pooja-bruno , I think it's fixed now. Please let me know if this way of solving it is good (I think some other multipart content types like alternative, digest, and so can take advantage of this....) or if you'd rather see an specific execution branch for mixed content-type.

Thanks!

[alfonso-presa]

Btw.... following my previous comment. If you'd like me to add an request.headers['content-type'].startsWith('multipart/') as an else if of the current "form-data" implementation so that it not only supports the mixed content-type, but it also does a best effort for other multipart subtypes just let me know and I'll do it right away.