Fix `ActiveRecordAdapter#ab_add_conversion` with implicit=true
Fix `ActiveRecordAdapter#ab_add_conversion` with implicit=true

Previously this added a participant for the alternative and recorded a
conversion for them, but did not record the alternative they had seen, meaning
the ab_counts were wrong.

This updates the implicit creation to record the alternative seen.

peterkovacs/vanity,assaf/vanity,peterkovacs/vanity,assaf/vanity,HealthyDelivery/vanity,HealthyDelivery/vanity,Futurelearn/vanity,Futurelearn/vanity,assaf/vanity,peterkovacs/vanity,HealthyDelivery/vanity,Futurelearn/vanity