Create PartParams class to allow multipart posts with JSON content and file upload at the same time

[jeremyisr]

Description

This PR aims to make it possible to multipart POST an IO and a JSON payload at the same time.
Fixes #769

Opened after discussions in #1014

Additional Notes

Example of API which requires this :
https://developer.concur.com/api-reference/receipts/endpoints.html#endpoint-post-a-receipt

cURL data and image:

curl -v -k -X POST https://us.api.concursolutions.com/receipts/v4/users/{USER ID FROM YOUR ID TOKEN} \
-H "Authorization: Bearer {YOUR ACCESS TOKEN}" \
-H "Content-Type:multipart/form-data" \
-H "link: <http://schema.concursolutions.com/{VALIDATION SCHEMA FROM SCHEMA ENDPOINT}.schema.json>;rel=describedBy" \
-F "receipt=<{PATH TO YOUR RECEIPT JSON};type=application/json" \
-F "image=@{PATH TO YOUR IMAGE};type={FILE MIME TYPE OF YOUR IMAGE}"

[technoweenie]

Hi @jeremy-israel, thanks for addressing my PR comments. I'll try to not to throw it all away again ;)

This looks good, but I have a few suggestions below.

[technoweenie]

Thanks for making those changes. I think this PR is ready to go, but there are some follow up tasks:

• Rename Faraday::UploadIO to Faraday::FilePart
• Update documentation for making multipart posts

[iMacTia]

Clean and well structured 👍

[technoweenie]

Hey, after seeing the class name in the code and docs, I'm slightly bothered by the plural ParamsPart name. It represents a single parameter of a multipart post request. A singular name like ParamPart will be more consistent with the renamed FilePart class too.

{ raw: Faraday::ParamPart.new(some_json, "application/json") }

I'm happy to make this quick change too if you don't want to bother :)

[jeremyisr]

Renamed ParamsPart to ParamPart.

[technoweenie]

Thanks again. Merging this for real now 🤘