write_csv does not invisibly return the dataframe

[tjmahr]

It returns the path, and write_delim returns NULL.

x <- readr::write_csv(iris, "iris.csv")
x
#> [1] "iris.csv"

x <- readr::write_delim(iris, delim = "\t", path = "iris.txt")
x
#> NULL

I've submitted a pull request to fix these.