[O] keyword :html-postamble-format seems to not be used

[Brady Trainor]

Looking at source of `org-html--build-pre/postamble`, I think `:html-postamble 
t` does check `org-html-postamble-format`, but I'd like it to check 
`:html-postamble-format` as well.

I encountered this problem as I'm trying to remove the validate string in the 
postamble, which `:html-postamble auto` will always include.

Not sure if there's a way to still keep the result of `:time-stamp-file` with 
`:html-postamble t`. That is, retain the inserted `Created: ...` string.

-- 
Brady