Feature Request: Add handling of multipart/mixed messages

[hhaensel]

Currently, only multipart/form-data is supported by multipart handling.

While this is probably the most important use case, also multipart/mixed is quite common, e.g. in REST APIs.
I recently struggled with batch requesting a SharepointOnline list and finally found my way through it. I thought, I could share my experience with the community and propose this PR #1100