Fix tests in XCode 10

[janhalousek]

Fixes failing tests in XCode 10

This MR is superset of #369 and probably resolves issues #368, #367 (haven't tested all cases)��.

The problem lies in casting nil variable to Any type, which seems to be a new thing in XCode 10 / Swift 4.2

class Bar {}
var foo: Bar? = nil

if let fooUnwrapped = foo as? Any {
    // this passes and fooUnwrapped is Optional(nil)
}

[jakubvano]

Thanks for the PR! 👍
I will try to release Swift 4.2 compatible version ASAP