Typo in is_writable_file

[Ralf Northman]

There seems to be a misplaced parenthesis in is_writable_file. The line with ifelse reads:

ifelse(file.exists(x, "unwritable", "nonexistent"))

but should read

ifelse(file.exists(x), "unwritable", "nonexistent")

This makes ifelse throw an error as it doesn't get enough arguments.

Thanks for a great package! /Ralf