Clarify spec about schemas with multiple types

In #1401 of swagger-core, @webron mentioned that specifying multiple types for a schema is not valid.

The 2.0 Spec says for a schema object:

The following properties are taken directly from the JSON Schema definition and follow the same specifications:
[…]

• type

JSON Schema validation says in section 5.5.2:

5.5.2. type

5.5.2.1. Valid values

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.

String values MUST be one of the seven primitive types defined by the core specification.

5.5.2.2. Conditions for successful validation

An instance matches successfully if its primitive type is one of the types defined by keyword. Recall: “number” includes “integer”.

I would interpret this as my type: ["integer", "string"] property being valid JSON Schema, and thus also valid Swagger.

While the "null" type name is not available in swagger due to it missing in the list of available types, the same consideration applies to something like "type": ["integer", "string"]. If this is not supposed to be valid, the sentence quoted above needs to be changed.

(Also, instead of referring to the “latest” version of JSON schema, I would explicitly name the version used, so the validity of Swagger documents doesn’t suddenly change whenever there is a new version of JSON schema.)

Related issues I found here: #297, #229, #403.

[RoyHP]

+1 for supporting array of types according to JSON Schema, we need this. I understand there’s a disclaimer indicating that JSON Schema is not entirely supported, but if you’re supporting the type key in the first place, it really ought to behave in the same manner.

[webron]

Not sure if there’s an existing ticket or not, would suggest searching the repo. Given the added support for oneOf, the need to support direct multi-types seems lower, but feedback is welcome.