Rather than unconditionally prepending `org-tag' to the existing text
property, I used `adjoin' here. Also, since the text property is
sometimes a list and sometimes just an atom, I make sure that we make an
atom into a list before feeding it to `adjoin'.

I've been applying this same change to the source every time I upgrade
org-mode. Since the last time, I can see that there was a change here to
strip nil property elements, but I'm guessing not too many others have
seen the same problem with malformed text property lists.

Please let me know if anyone would like more detail on the malformed
text property lists I've seen arise without this patch in place.

-- 
Steven E. Harris