Legacy and non-legacy mungebits fail in certain scenarios

[robertzk]

dv <- syberiaMungebits::drop_variables
class(dv) <- c("legacy_mungebit_function", class(dv))
x <- tundra_container$new("foo", munge_procedure = list("drop vars" =
  mungebits:::mungepiece(mungebits:::mungebit(dv), c(1,2)),
  "double" = mungebits2::mungepiece$new(mungebits2::mungebit$new(function(df) {
  df[[1]] <- 2*df[[1]]; df })))
  )
x$train(iris)
x$predict(iris) # This fails!