change ale to use value of g:ale_shell regardless of what it is

test/test_prepare_command.vader

@@ -4,7 +4,7 @@ Before:
 
 After:
   Restore
-  let g:ale_shell = &shell
+  let g:ale_shell = v:null
 
 Execute(sh should be used when the shell is fish):
   if !has('win32')