fix JSONEncodable type test for Array and Dictionary

[toco]

Fix a bug where when an array or dictionary was directly declared with element/value type JSONEncodable, the test if the element/value is a JSONEncodable did fail.

E.g.

let someArray:[JSONEncodable] = [1,2,3]
someArray.elementsAreJSONEncodable() // false

Since the array elements are explicitly conforming to the JSONEncodable protocol it should return yes.