Hi

The vi-backward-kill-word widget does not seem to respect WORDCHARS:
```
frustcomp% zsh -f
frustcomp% bindkey "^W"
"^W" vi-backward-kill-word
frustcomp% mkdir /tmp/foo_bar
frustcomp% ls /tmp/        
frustcomp% WORDCHARS='*?-.[]~=/&;!#$%^(){}<>'
frustcomp% ls /tmp/foo_bar
```
typing ^W at this point will delete 'foo_bar'.
I expect it to only delete 'bar', though, as that is what backward-kill-word does.

Is this a bug or intended?
If intended: is there a different way of adjusting the behaviour of vi-backward-kill-word?

Thank you

henk