Fix border re-rendering

[vadimdemedes]

Fixes #340.

In the following scenario, border wouldn't be rendered after the update happens:

const Test = () => {
	const [borderColor, setBorderColor] = useState();

	useEffect(() => {
		setBorderColor('green');
	}, []);

	return (
		<Box borderStyle="round" borderColor={borderColor}>
			<Text>Hello World</Text>
		</Box>
	);
};

It was happening because Ink's reconciler passes down only props that were changed during an update (https://github.com/vadimdemedes/ink/blob/master/src/reconciler.ts#L160) and in this case, borderStyle remained unchanged so it was missing when Ink drew a layout. If Ink doesn't see borderStyle, it's not going to render the border - https://github.com/vadimdemedes/ink/blob/master/src/render-border.ts#L7.

To fix it, I modified reconciler's prepareUpdate function to always include borderStyle and borderColor if at least one of them is present. Now if borderStyle stays unchanged, but borderColor changes, both props will be included in the update.