If two fields have the same name but different explicit name spaces,
treat as non-conflicting. This allows parsing common XML formats
that have ns1:tag and ns2:tag in the same XML element.
Fixes #4691.

Allow setting the default name space for unadorned tags, by
writing to Decoder.DefaultSpace. This allows turned the job of
parsing common XML formats that have tag and ns2:tag in the
same XML element into the first case by setting DefaultSpace="ns1".
Fixes #3703.

Use name space attributes when decoding.
Attach name space to attributes when encoding.
Could be done with fewer annotations, but semantically correct as is.
Fixes #3526.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7227056