Fixed multipart support for array type request body properties

[neidhart-orlich-bt]

I have the very strong impression that request body properties of type 'array' in multipart requests were serialized wrong. For such a property a single part was created with a string representation of the array.
I fixed that so that now multiple parts with the same part name (the key of the array property) are created instead which contain the respective element's content.

Description

In src/execute/oas3/build-request.js I extracted the current transformation of request body root properties in a method and repeated the procedure for the elements of array type properties. I made sure that this happens only once - so arrays that are contained by another array are transformed like before - i.e. converted to a string.

In src/http.js I added special handling for arrays so that there won't be now a single part for the array but multiple parts for each array element all having the same name: the key of the array property.

Motivation and Context

Similar like in issue #1426 I wasn't able to send a request containing an array with multiple files to our server. Soon I found out that this is a general issue with array type properties.
I already created a question in the SmartBear community forum without getting an answer but it documents the issue very well:
https://community.smartbear.com/t5/Swagger-Open-Source-Tools/Can-Swagger-UI-for-Openapi-v3-handle-arrays-in-multipart/td-p/193648

How Has This Been Tested?

Sorry I have no idea how to test this but if you help me to set up an environment I will test it gladly.

Also please note that I am a java developer and don't know much about node.js.

Screenshots (if appropriate):

Types of changes

• No code changes (changes to documentation, CI, metadata, etc)
• Dependency changes (any modification to dependencies in package.json)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I am unsure about all of these but happy to help if you tell me how.

[neidhart-orlich-bt]

Some tests seem to fail... If somebody would be so kind as to help me out set up an environment where I can run them locally I will try to understand what's going on

[tim-lai]

@neidhart-orlich-bt Thank you for your PR. I am closing this because it is superseded by PR #1527