When doing an attribute append, drop the + in the attribute name.

We were accidentally including it, so the attribute that we
were supposedly updating was <attribute>+ instead of just
<attribute>.
Fix the appending attribute updater.

Hadn't tested it, an old variable name was still in there.
Appending attribute updater creates the attribute if it doesn't exist.

Before it would end up with "undefined <new value>" as the
value of the attribute if the attribute did not already exist.