Zsh includes directories in tab completion

["Saverio M." <saverio.pub2@gmail.com>]

Hello,

On my system, zsh tab-completion includes directories present in the 
path, in addition to files, which is confusing and error prone.

Is this intended? I think it isn't, but I didn't find a way to disable 
it (there's a lot of documentation around, but the posts I've found talk 
about tab completion customization).

To be specific, I had Oh My Zsh installed, but I've removed it, and I've 
also removed .zshrc to be sure I'm running a vanilla setup.

This is the test case:

```sh
$ echo $ZSH_VERSION
5.4.2

$ echo $ZDOTDIR

$ cat $HOME/.zshenv
cat: /home/myuser/.zshenv: No such file or directory

$ cat $HOME/.zprofile
cat: /home/myuser/.zprofile: No such file or directory

$ cat $HOME/.zshrc
cat: /home/myuser/.zshrc: No such file or directory

$ cat $HOME/.zlogin
[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm" # 
Load RVM into a shell session *as a function*

$ cat $HOME/.zlogout
cat: /home/myuser/.zlogout: No such file or directory

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/myuser/bin:/home/myuser/code/scripts:/home/myuser/.rvm/bin

$ ls -ld $(find ~/code/scripts -name 'myscript*')
drwxr-xr-x 1 myuser myuser  752 Dec  2 18:39 
/home/myuser/code/scripts/myscript
-rwxrwxr-x 1 myuser myuser 4571 Dec  2 18:40 
/home/myuser/code/scripts/myscript.rb

$ myscript<TAB>
myscript     myscript.rb
```

Can anybody shed some light?

Thanks!
Saverio