cannot speak if joining a chatroom with special characters nickname

[vetetix]

When joining a chatroom with the nickname "A²", I cannot speak on it (the "send" button is grey, and i cannot write in the text input field. However, I can select a smiley and send it.

My nick is automatically changed on connection to "A2", meaning that "A²" isn't a valid nickname.

When I change my nick from "foo" to "A²" in a chatroom I'm allready connected to, it changes my nick to "A2", and i can speak as usual.

The problem occurs on Windows and Linux (Ubuntu), both with gajim 0.11.4, and on every server I tested it on.

[asterix]

(In [9589]) string prep nickname before joining. Fixes #3770