encoding/xml: handle time.Time as recognized type

The long term plan is to define an interface that time.Time
can implement and that encoding/xml can call, but we are
not going to try to define that interface before Go 1.
Instead, special-case time.Time in package xml, because
it is such a fundamental type, as a stop-gap.
The eventual methods will behave this way.

Fixes #2793.

R=golang-dev, r, r, n13m3y3r
CC=golang-dev
https://golang.org/cl/5634051