[lua] Fix Dialog:file{ save=true } so 'save' field must be =true

Before this change if we used Dialog:file{ save=false } it would show
the save version of the dialog, which didn't make sense.

src/app/script/dialog_class.cpp

@@ -777,7 +777,7 @@ int Dialog_file(lua_State* L)
     lua_pop(L, 1);
 
     int type = lua_getfield(L, 2, "save");
-    if (type == LUA_TBOOLEAN) {
+    if (type == LUA_TBOOLEAN && lua_toboolean(L, -1)) {
       dlgType = FileSelectorType::Save;
       title = "Save File";
     }