rooms are created in prosody in their urlencoded form, eg täst
becomes t%c3%a4st@conference.jitsi.example.org
As local params = formdecode(query) contains an urldecode, we
need to reencode the room name so that the room can be found in
prosody.
Closes: #10525
Signed-off-by: Christoph Settgast <csett86@web.de>