You cannot pass arbitrary strings to send(). They must be DOM elements. You can use Strophe's xmlElement() function to generate these if you like, or use the Strophe builder stuff.
For example:
Code:
Connection_.send($presence().c('priority').t(1));
would do the same thing your code is trying to do.
If you want to see how these DOM elements are built behind the scenes, you can look at the strophe.
js source.