encoding/xml: add whitespace normalization from spec

What version of Go are you using (go version)?

go version go1.8.3 darwin/amd64

What operating system and processor architecture are you using (go env)?

darwin/amd64

What did you do?

for attribute values, all whitespace characters (#x20, #xD, #xA, #x9), should be normalized to a space character (see https://www.w3.org/TR/REC-xml/#AVNormalize)

go program used to test: (https://play.golang.org/p/mCRrxvyh25)

package main

import (
	"encoding/xml"
	"fmt"
	"strings"
)

func main() {
	decoder := xml.NewDecoder(strings.NewReader(`<a p1="v1

v1"/>`))
	t, _ := decoder.Token()
	if e, ok := t.(xml.StartElement); ok {
		fmt.Printf("%q", e.Attr[0].Value)
	}
}

What did you expect to see?

"v1  v1"

What did you see instead?

"v1\n\nv1"

Tested the same with xmllint program:

$ cat test.xml
<x p1="v1

v2"/>
$ xmllint test.xml
<?xml version="1.0"?>
<x p1="v1  v2"/>