exp/html: tokenize attributes of end tags

exp/html: tokenize attributes of end tags

If an end tag has an attribute that is a quoted string containing '>',
the tokenizer would end the tag prematurely. Now it reads the attributes
on end tags just as it does on start tags, but the high-level interface
still doesn't return them, because their presence is a parse error.

Pass 1 additional test.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6457060