Further improve "actual code over markup"

[jankowtf]

In unit test for setReactiveS3:

## Markup and actual code //
  ## Actual code always takes precedence over markup!
  where <- environment()
  setReactiveS3(id = "x_1", value = 10, where = where)
  setReactiveS3(id = "x_2", 
    value = function() {
      ## object-ref: {id: x_2, where: where, as: REF_1}
      ref_1 <- where$x_1

      ## Do something with the references //
      ref_1 * 2
    }
  )
``