[bug] text-face-names containing &s aren't escaped properly

http://www.dafont.com/el-font-bubble.font is called "El&Font Bubble Regular".  However, when using that name, Mapnik fails to load it because the value in the resulting stylesheet isn't escaped.

Returning `xmlvalue` in `tree.Quoted#toString()` (vs. `this.value`) seems like it would work, but it breaks various other tests and doesn't seem quite right.