encoding/xml: correctly escape newline, carriage return, and tab

The generated encodings are those from
http://www.w3.org/TR/2000/WD-xml-c14n-20000119.html#charescaping

The change to the decoder ensures that we turn 
 in the
input into \r, not \n.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6747043
2 files changed