Support for content-type in each of multi-part rquest

[nishantrayan]

This allows users to specify content-type in payload like,
RestClient.post(url, {data: "something", content_type: "text/plain"})

Some Rest servers check for content type in each part before taking doing anything with the data in part.
This option will only be honored with multipart option enabled.

[aliaksandr-trush]

+1 for this enhancement

[jlambert121]

I ran into this today - I have a submit I needed to do with two parts, each with a different content type. The way I read this, it appears to only allow setting the content-type if there is a single part, is that correct?

[nishantrayan]

Merged the test cases. It can be merged to the main repo.

[aliaksandr-trush]

Hello, @ab can you merge the pull request?

[abotalov]

Currently params argument is a hash where keys are part names and values are bodies of these parts.
This pull request makes :content_type key being treated specially so it will represent not a part name but Content-Type that has to be applied to all regular parts. In particular, it doesn't allow different parts to have different Content-Types.
IMO it's not a good design decision. Also it's not quiet backwards compatible.