Fix rake task to manually assign shops

@wvanbergen @pseudomuto 
/cc @richgilbank 

Fixes #2 

The rake task was setting `originally_created_at` to false, instead of nil. That made `should_store_assignment?` return false, because `assignment.returning?` was true, because `@returning` is set to `!originally_created_at.nil?`.

BTW, I have *no idea* what the `returning?` method is supposed to mean.