encoding/xml: improve []byte handling

encoding/xml: improve []byte handling

Marshalling of []byte in attributes and the general
marshalling of named []byte types was fixed.

A []byte field also won't be nil if an XML element
was mapped to it, even if the element is empty.

Tests were introduced to make sure that *struct{}
fields works correctly for element presence testing.
No changes to the logic made in that regard.

R=rsc
CC=golang-dev
https://golang.org/cl/5539070