Nested errors are still not serialized

[fregante]

I was thrown off by this test:

serialize-error/test.js

Lines 129 to 136 in 6f6102f

	test('should serialize nested errors', t => {
	const error = new Error('outer error');
	error.innerError = new Error('inner error');
	const serialized = serializeError(error);
	t.is(serialized.message, 'outer error');
	t.is(serialized.innerError.message, 'inner error');
	});

This is incorrect. The test is not testing whether the property was serialized, but whether it exists. The nested error is there still as an Error instance.

• See Actually serialize nested Errors #73