Support POST multipart form content

[magnayn]

Per Issue #12 - having user upload needs support for multipart form content.

Credit for the code here is @artipop from that thread, I just packaged it and gently refactored to the current code.

Could likely use a test -- I wasn't sure how to influence the infrastructure in the ones I could see to send different content-types; if someone could give me a hint I'd be happy to add..

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[sdelamo]

Yes, it would need a test and you would need to sign the CLA as well.