Fix #458

[chainsawriot]

The S3 method can be dispatched. But the problem is that it can't be tested with testthat. Why?

[chainsawriot]

Concretely, the code in #458 works. But with testthat, the custom S3 method cannot be dispatched. This case

rio/tests/testthat/test_extensions.R

Line 10 in 727d50f

##expect_true(is.data.frame(import('iris.custom')))

[chainsawriot]

r-lib/testthat#720

[chainsawriot]

rio/R/utils.R

Lines 66 to 68 in 727d50f

	if (is.na(info$format)) {
	return(input)
	}