Allow asking for any variant on an ab_test

If ab config changes variant configuration for an ab test,
ab lib used to throw errors if `ab_test.some_variant?` code
was invoked. Now it returns false.

Java/Objective-C implementations seem to behave in similar way.

cc @mmozuras 
> In general analytics dashboards should show exposure numbers/variant which should be enough to detect issues?

:+1: yeah, that's similar to what I had in mind with the second message.