This parser actually supports way more than is needed for RSP proper,
as mt-daapd was piggybacking on the RSP queries for its smart playlists.
As I don't have (yet?) the RSP specs, better safe than sorry here. This
will be revisited at some point. Or not.
There's no proper way to add attributes to the root XML element in mxml;
the best way to do that is to create the root element like we would
create any other element and put the whole "? ... ?" stuff in there.
The SoundBridge does not support empty XML tags (<foo/>) and treats anything
containing such tags as invalid. Adding an empty string to the element works
around that, in that mxml won't produce empty tags in this case.
Hackish, but dealing with a hackish implementation on the other side :)
iTunes and Roku devices do not encode + as %2B in the query string and
do not encode space as + either in the query string (though at least the
Roku encode space as %20 everywhere). This needs to be worked around or
browse queries fail to parse because + was decoded as space when the query
really needs a + character.
