exp/html: improve InHeadIM
exp/html: improve InHeadIM

Clean up the flow of control, and add a case for doctype tokens (to
ignore them).

R=nigeltao
CC=golang-dev
http://codereview.appspot.com/6069045

Committer: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old