Then return nil as parameter

[vlchapaev]

Stubbed function should be allow tor return 'nil'. Suggest to make result param optional.

Mirage/Mirage/Stub.swift

Lines 45 to 56 in a5d3150

	/// Returns new result instead of default
	///
	/// - Parameter result: New result.
	/// - Returns: A stub for chained call.
	@discardableResult
	public func thenReturn(_ result: Any) -> Stub {
	let stubAction = StubAction { (_) -> Any? in
	return result
	}
	actions.append(stubAction)
	return self
	}