changed encoding to fix XML parsing issues

[klinskyc]

ISO encoding led to XML parsing issues for artists with non english character names (e.g Jhené Aiko). Switching encoding to UTF is an easy fix.

[volontarian]

You get the build green if you write (solution found here: http://www.germane-software.com/software/rexml/docs/tutorial.html):

body.encode(Encoding.find("UTF-8"), :undef => :replace).unpack("C*").pack("U*")

But I think it's better to use UTF8 encoding the way @klinskyc did it, remove the spec example and raise errors here instead of returning a wrong JSON. So with ISO-8859-1 it raises no exception but it returns a hash for [track]...[/track] but not the expected array for [recenttracks][track]...[/track][/recenttracks].