Fix layout context prop resolving for boolean types (#567)

[atmelmicro]

Allows boolean props to propagate when layout context is false

Closes #567

[adamkudrna]

Does it make any sense to have a test for it? Possibly with an example similar to that in the original issue #567.

[mbohal]

I think Ill leave it up to @atmelmicro to decide.

It is one simple condition, so I think that maybe just a comment would work:

// We need to test:
//  * `false` - for when the `contextValue` is boolean
//  * `null` - for when the `contextValue` is non-boolean
if (contextValue === false || contextValue === null) {

However test would communicate it just as good and it would test it on top.

I have no preference, really, but I agree with @adamkudrna that one or the other should be added.