Remove extra space in an empty tag for format_raw

When using format_raw the space in the empty tag (<node />) is the only
character that does not have to be there; so format_raw almost results in
a minimal XML but not quite.

It's pretty unlikely that this is crucial for any users - the formatting
change should be benign, and it's better to improve format_raw than to add
yet another flag.

Fixes #87.