Use file-name-as-directory to add a trailing slash

`default-directory` should end with slash as you can see via the command `(describe-variable 'default-directory)`, otherwise `start-process` and `call-process` doesn't work correctly, because `unhandled-file-name-directory` is applied to `default-directory`.

Here is an example:

```
(defun inf-ruby-pwd (dir) 
  (interactive "D") 
  (let ((default-directory dir))
    (run-ruby "pwd" "pwd")))
(call-interactively 'inf-ruby-pwd)
```

If you specify "/tmp", which doesn't have a trailing slash, the output buffer will be like below:

```
/

Process pwd finished
```