Digestor: pass the missing "keys" param to LookupContext

Related to #14243 & #14242

Took me a while to debug this. The problem was that Digestor didn't pass the `keys` param to `LookupContext` (actually passing `options` as `keys`), so variants were lost in between.

I don't consider it ready, because I haven't figured out a way to test it properly.

Also, there's one catch: if `show.html+phone.erb` exists, it will be rendered, but if doesn't, lookup will fall back to `show.html.erb`. I'm not sure if that's a desirable behavior.

/cc @dhh @strzalek @jeremy