exp/html: discard tags that are terminated by EOF instead of by '>'

exp/html: discard tags that are terminated by EOF instead of by '>'

If a tag doesn't have a closing '>', it isn't considered a tag;
it is just ignored and EOF is returned instead.

Pass one additional test in the test suite.

Change tokenizer tests to match correct behavior.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6454131