Fix #unwrapType by recursively unwrapping to find root scalar or composite type

[imranolas]

type Query {
	threads: [Thread!]
}

type Thread {
	id: ID!
	title: String!
}

Given the schema above the following threads selection set would be incorrectly identified as a list.

query {
	threads {
		id
		title
	}
}

This change recurses through the linked types, unwrapping each one until the root non-wrapping type is found.