Failing NSE example

```r
mp <- mungepiece$new(mungebit$new(transform), train_args = alist(sep = Sepal.Length * 2))
expect_equal(mp$run(iris)$sep, iris$Sepal.Length * 2)
```

fails while

```r
expect_equal(mungebit$new(transform)$run(iris, sep = Sepal.Length * 2)$sep, iris$Sepal.Length * 2)
```

works.

See [this PR](https://github.com/syberia/mungebits2/pull/43)
