expect_identical shouldn't always call all.equal()

[wch]

This happens now due to some changes in R-devel:

library(testthat)
e <- new.env()
class(e) <- "foo"
expect_identical(e, e)
# Error in as.vector(x, "list") : 
#   cannot coerce type 'environment' to vector of type 'list'

But it might get fixed in R-devel...

[hadley]

Hmmmm, this might need some fundamental changes to testthat - currently both the failure and success messages are generated regardless of which occurs.

[hadley]

Or I could just make the error message less informative.

[wch]

A less-informative error message seems fine to me.