[R] Nightly builds (R 3.5 and 3.6) failing due to field refs test

[thisisnic]

Describe the bug, including details regarding any error messages, version, and platform.

The test-r-versions nightly build is failing on R 3.5 and 3.6 due to a test introduced in #19706.

On those builds, the second column in the input data.frame is being converted into a dictionary instead of a string. This is due to the fact that prior to R 4.0, the default value of the stringsAsFactors parameter in data.frame() was TRUE, but since then has been changed to FALSE.

Test body:

test_that("Nested field from a non-field-ref (struct_field kernel)", {
  x <- Expression$scalar(data.frame(a = 1, b = "two"))
  expect_true(inherits(x$a, "Expression"))
  expect_equal(x$a$type(), float64())
  expect_error(x$c, "field 'c' not found in struct<a: double, b: string>")
})

Test result:

══ Failed tests ════════════════════════════════════════════════════════════════
── Error ('test-expression.R:154'): Nested field from a non-field-ref (struct_field kernel) ──
Error: field 'c' not found in struct<a: double, b: dictionary<values=string, indices=int8, ordered=0>>
Backtrace:
    ▆
 1. ├─testthat::expect_error(x$c, "field 'c' not found in struct<a: double, b: string>") at test-expression.R:154:2
 2. │ └─testthat:::expect_condition_matching(...)
 3. │   └─testthat:::quasi_capture(...)
 4. │     ├─testthat (local) .capture(...)
 5. │     │ └─base::withCallingHandlers(...)
 6. │     └─rlang::eval_bare(quo_get_expr(.quo), quo_get_env(.quo))
 7. ├─x$c
 8. └─arrow:::`$.Expression`(x, c)
 9.   └─arrow:::get_nested_field(x, name)

Component(s)

R