Fixes #772

This fixes  #772 which makes using PIP_EXISTS_ACTION throw an error.

However I noticed that in the current state pip exists actions are never called 

In download.py this code:

```python
   if download_dir and not already_downloaded:
        _copy_file(temp_location, download_dir, content_type, link)
```

Prevents `_copy_file` from being called when files are already downloaded and thus
exists actions are not taken into account and shows instead this message:
>File was already downloaded <package>

I was not sure if this is the right behavior for exists_action can someone explain how 
it is supposed to work. I could add a better explanation of this option in the docs.