virtualenvwrapper: set $WORKON_HOME if undefined

This uses the default that virtualenvwrapper.sh would set if it was called. If the user
changes its value after the plugin is loaded, the plugin will work all the same.

Fixes #6882
Closes #6870
Closes #6883

plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh

@@ -7,6 +7,7 @@ if (( $+commands[$virtualenvwrapper_lazy] )); then
     unsetopt equals
     virtualenvwrapper=${${virtualenvwrapper_lazy}:c}
     source ${${virtualenvwrapper_lazy}:c}
+    [[ -z "$WORKON_HOME" ]] && WORKON_HOME="$HOME/.virtualenvs"
   }
 elif (( $+commands[$virtualenvwrapper] )); then
   function {