`is_writable_file` has minor syntactical error, here is fix

[Mike Williamson]

Example:

is_writable_file(<any_file_string_you_wish>)
Error in ifelse(file.exists(x, "unwritable", "nonexistent")) :
  argument "yes" is missing, with no default

Looking at the actual code where this error is created (line 9 of function):

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

This should instead be:

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

Note the movement of one closing parenthesis.

assertive.files version:

Version: 0.0-2

Regards, Mike Williamson