[swift5]: support arrays in multipart/form-data

Created by: kalinjul

For an API POST request with content type "multipart/form-data" and an parameter of type: array, the generated Implementations for Swift (Alamofire/UrlsessionImplementation) would crash. As array parameters are simply put inside a multipart/form-data request as multiple entries with same key, this PR adds a for loop when we encounter an array.

A bit hard to see in the github diff: I only added

for v in (v as? Array ?? [v]) {

in both implementations.

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@4brunu: mentioning you for review although @wing328 reviewed my last swift PR.