Check for property via selector before using value(forKey:)

[ilyapuchka]

Resolves #231

[djbe]

Ideally we'd be able to log some sort of warning instead of silently continuing. More often than not, I prefer the compiler/runtime to tell me if I did something wrong instead of just doing it's own thing.

What's the general consensus on this? Error out (current behaviour), or silently continue? @AliSoftware @kylef @yonaskolb

[ilyapuchka]

This is consistent with current behavior for undefined properties. If we want to revisit it, which I agree we, I think better to do it separately.

[djbe]

If it's current behaviour, then it's good for me.

[mbrandonw]

Thanks for taking care of that so quickly @ilyapuchka!