JsonSchema.type properties have not allowed an array of string now. type properties should receive a string and array of a string by a spec of JSON Schema.

https://github.com/json-schema-org/json-schema-spec/blob/master/schema.json#L151-L161

Test cases of tv4 contain an array of string in type properties.

https://github.com/geraintluff/tv4/blob/master/test/tests/01%20-%20Any%20types/01%20-%20type.js#L10-L22

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