Crashfix: 'From' jid with a non-ASCII char sending a command

Fixes a crash occurring when a remote jid with a
non-ASCII character issued a XEP-0050 Ad-Hoc
command.

Specifically, jid=str(...) was changed to
jid=unicode(...) in commands.py. No side effects
observed.

It was sufficient to have a user called
"mačak@server.tld" to issue a command request
and the component would crash.