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 
I'm still ruminating on the following scenario:

Two variants, on and off.
You write code ~`unless ab.test.off?`.
Now, if you mistype `off`, you kinda ruin the test if you run it for all users.

It's entirely realistic scenario, but I'm still slightly miffed. Let's talk about it a little bit more on Monday?