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>

tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang