The following testcases now pass:

`<a href=x` tests that we do not error on partial unquoted attrs.
`<a href=x ` tests that spaces do end unquoted attrs on spaces.
`<a href=''` tests that we recognize the end of single quoted attrs.
`<a href=""` tests that we recognize the end of double quoted attrs.

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/4932051