order_by does not handle something that is not a call

[romainfrancois]

order_by(10:1, cumsum(1:10))
#  [1] 55 54 52 49 45 40 34 27 19 10
> cx <- cumsum(1:10)
> order_by(10:1, cx)
Erreur : is.call(call) n'est pas TRUE

This causes things like this to fail:

right <- mutate(scrambled, running = order_by(year, cumsum(value)))

because cumsum is substituted internally with the hybrid evaluator.

It probably means that I need to come up with an internal hybrid version of order_by.