Handle multiple values in case/when ...

[presidentbeef]

Do not warn about these cases:

case x
when 1, 2, 3
  maybe_dangerous(x)
end

and

y = [1, 2, 3]

case x
when *y
  maybe_dangerous(x)
end

(In the future, it might make sense to assign a union of the values (e.g. x = 1 || 2 || 3), but not doing that now because it has performance implications and not sure it matters for security issues.)

Fixes #1730