Name assignemnt is more consistent

[marberts]

https://bugs.r-project.org/show_bug.cgi?id=18558

Now

x <- 1:3
attr(x, "names") <- setNames(letters[1:3], x)
identical(names(x), letters[1:3])

is TRUE.

This also fixes the malformed factor error with

names(structure(1:3, names = factor(1:3)))

and makes it so

names(x) <- as.Date("2017-01-01")
identical(names(x), names(structure(x, names = as.Date("2017-01-01"))))

is TRUE