feature/unions for array or objects

[xddq]

originally done by @smeijer
here. just created new PR
cause I somehow could not commit to the original one. I think the reason is
that the pr creator account and the fork live in different accounts. But it is
just an assumption.

Summary

This adds support for unions that have an array or object in them, like in the schema below.

Other unions like [string, null] were already supported. Unions with arrays [array, null] and objects [object, null] were not. Tests covering these new cases are included.

{
  type: "object",
  properties: {
    nullableObject: {
      type: ["object", "null"],
      properties: {
        name: { type: "string" },
      }
    },
    nullableArray: {
      type: ["array", "null"],
      items: {
        type: "string"
      },
    },
  }
});

I'm making this change, because we have schemas with nullable arrays and objects. Without this change, those schemas cannot be parsed.

Fixes #43