Use bytes charset for multipart input

[krlmlr]

Closes #75.

My fork of @sellorm's demo at https://github.com/krlmlr/plumber-uploader/tree/f-multipart contains an example of reliably processing multipart data using mime::parse_multipart(). This PR adds special treatment for multipart input.

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PR task list:

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[meztez]

See #532 as it opens up a whole lot more possibility and parse multipart correctly. I don't think we should let the end user deals with the bytes, it should just work ™.

A generalized approach to input parser would be more friendly to developer.

[schloerke]

@krlmlr Thank you for getting the conversation started! I believe @meztez has covered this in #532.

I ran your example with the current master branch (which has #532 merged) and I believe it worked as expected. I ran it with the current CRAN version and had unexpected behavior involving queryString.

Closing this PR.