html/template: reenable testcases and fix mis-escaped sequences.
html/template: reenable testcases and fix mis-escaped sequences.

Tighter octal parsing broke some tests and were disabled in
http://codereview.appspot.com/5530051

Those tests were broken.  The CSS decoder was supposed to see CSS
hex escape sequences of the form '\' <hex>+, but those escape
sequences were instead being consumed by the template parser.

This change properly escapes those escape sequences, and uses
proper escaping for NULs.

R=golang-dev, rsc, nigeltao
CC=golang-dev
http://codereview.appspot.com/5529073

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