Fix shwordsplit bug when a basedir contains spaces

The `shwordsplit` option affects all variables and we only need to split
the `$EDITOR` variable. Because of that, using `${=spec}` is a much better
alternative.

More info at
http://zsh.sourceforge.net/Doc/Release/Expansion.html#index-SH_005fWORD_005fSPLIT_002c-toggle