Support for array in type keyword

[rodcloutier]

According to the json schema upon which OpenAPI spec relies, type keyword can be either a string or an array.

The value of this keyword MUST be either a string or an array. If it
is an array, elements of the array MUST be strings and MUST be
unique.
https://datatracker.ietf.org/doc/html/draft-wright-json-schema-validation-00#section-5.21

Using this feature in an OpenAPI spec v2 prevents parsing

The following snipped

...
  "definitions": {
    "budgetCode": {
	"type": [
		"string",
		"null"
	],
	"format": "nullable string"
}, 
...

produces

2022/06/30 14:02:18 Failed de decode OpenAPI Spec: failed to unmarshal property "definitions" (*map[string]*openapi3.SchemaRef): failed to unmarshal property "properties" (*openapi3.Schemas): failed to unmarshal property "type" (*string): json: cannot unmarshal array into Go value of type string

[fenollp]

Indeed you're right. From https://github.com/OAI/OpenAPI-Specification/blob/main/versions/2.0.md#schema-object I arrive at a similar passage of https://datatracker.ietf.org/doc/html/draft-fge-json-schema-validation-00#section-5.5.2.1

Same for OpenAPIv3.1 https://datatracker.ietf.org/doc/html/draft-bhutton-json-schema-00#section-7.6.1

In OpenAPIv3.0 type can only take a single string value per https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#properties and the code in ./openapi2 relies on that implementation.
So here's what needs to happen:

• Clone the struct *openapi3.SchemaRef in ./openapi2, making sure type can take both string and []string and removing non-v2 fields if any.
• Replace the corresponding code in openapi2_conv.go so the newly introduced struct is used.

When converting type from v2 to v3 you can use this equivalence:

'Legal in v2':
  type: [boolean, string]

'Legal in v3.0':
  oneOf:
  - {type: boolean}
  - {type: string}

Please send PRs my way! :)

[rodcloutier]

@fenollp Working on a PR.

Not too sure at the moment what should be the behaviour of the code at https://github.com/getkin/kin-openapi/blob/master/openapi3/schema.go#L655 where we must validate the format associated with the type and we have multiple types. Do we have any reference to this case in the specs?

I guess the format should not be specified if multiple types are provided...

[TristanSpeakEasy]

@rodcloutier have you made any more progress on this? @fenollp any advice for rod?

Depending on the status of this, it might be something I will need to pick up soon if we aren't close to a solution?

[rodcloutier]

@TristanSpeakEasy If you wish you can take over the work. I haven't had time to make any progress.