Bug 404419 - XML not displayed. Adds a trivial reftest that checks that XML pretty-printing happens for XML documents. r=peterv, a=itsatest

layout/reftests/bugs/reftest.list

@@ -473,3 +473,4 @@ fails == 386310-1d.html 386310-1-ref.html
 != 404030-1-notref.html 404030-1.html
 != 404030-1-notref2.html 404030-1.html
 == 404301-1.html 404301-1-ref.html
+!= data:application/xml,<foo/> data:text/plain, # bug 404419