Fixed Email with subject doesn't work

That is not really a bug. "mailto" links are just not allowed in the URL tag. But you can modify the code to make it work. See this thread:
That is a dirty hack to the base code, hence why is bug. It would be overwritten if xenforo was updated or re-uploaded. Also this is something ultimately basic, an addon for it is redundant.

Small bug, but it's still a bug.