[Bug] Fix multipart schema inference for allOf/anyOf/oneOf of primitive types and non-binary arrays

[czechboy0]

Motivation

As I started testing the multipart generation on real-world projects, I discovered two bugs:

• allOf/anyOf/oneOf of primitive types (such as string) were encoded as JSON instead of a raw string (aka HTTPBody), which was wrong
• arrays of non-binary and arrays of binary elements were treated inconsistently

Modifications

Fixed the bug by refactoring the inferrence logic a bit.

Result

Now e.g. an anyOf of a string still gets encoded as a primitive type, not JSON.

Test Plan

Added a unit test for this logic with a few test cases, easier to debug this way.