exp/template/html: fix bug in cssEscaper
cssEscaper escapes using the CSS convention: `\` + hex + optional-space
It outputs the space when the escape could be followed by
a hex digit to distinguish a "\na" from "\u00aa".
It did not output a space when the escape is followed by a space
character so did not distinguish "\n " from "\n".
Currently when doing lookahead, it does not distinguish spaces that
will be escaped later by the same function from ones that will not.
This is correct but suboptimal.
R=nigeltao
CC=golang-dev
https://golang.org/cl/
5306042