Update to go 1.12 version of golang.org/x/net

Change-Id: I3f2fa5d7b6811c9eca58992318d19e44ec9131fd

vendor/golang.org/x/net/html/node.go

@@ -177,7 +177,7 @@ func (s *nodeStack) index(n *Node) int {
 // contains returns whether a is within s.
 func (s *nodeStack) contains(a atom.Atom) bool {
 	for _, n := range *s {
-		if n.DataAtom == a {
+		if n.DataAtom == a && n.Namespace == "" {
 			return true
 		}
 	}