Ensure Ops falls back to base behaviour

[hadley]

Fixes #320

I also tried this:

chooseOpsMethod.S7_object <- function(x, y, mx, my, cl, reverse) {
  S7_inherits(x) && S7_inherits(y)
}

But that leaves us with (e.g.) incompatible methods ("Ops.S7_object", "Ops.factor") for "+".