Show whitespace in default prop

[sapegin]

Now if prop’s default value contains only whitespace it renders invisible. I’m not sure about the best solution but at least we can quote whitespace-only values or render spaces as something like ␣.

[aaronjensen]

How about quoting always? JSON.stringify essentially.

[sapegin]

Would look too noisy in my opinion. But at least quotes should be grayish.