Fix #833 - Do not open windows on save when the option is off

autoload/ale/list.vim

@@ -18,7 +18,7 @@ function! s:ShouldOpen(buffer) abort
     let l:saved = getbufvar(a:buffer, 'ale_save_event_fired', 0)
 
     return (type(l:val) == type(1) && l:val == 1)
-    \   || (l:val ==# 'on_save' && l:saved)
+    \   || (type(l:val) == type('') && l:val ==# 'on_save' && l:saved)
 endfunction
 
 function! ale#list#SetLists(buffer, loclist) abort