encoding/xml: does not process multiple namespace definitions correctly

[jordan2175]

The unmarshal / marshal functions do not handle multiple xmlns definitions. See:

https://play.golang.org/p/KTemXABrFb

If you remove all but one of the namespace definitions, it will sort of work.

[mikioh]

Dup of #7113

@jordan2175, feel free to send a CL; http://golang.org/doc/contribute.html

FWIW, there were CL 93320043, but not completed due to more important tasks. I think you can ask him to take over https://codereview.appspot.com/93320043.

[rogpeppe]

The example is wrong. If you replace the namespace prefixes in the struct
tags with full URLs, then it seems to work OK.

https://play.golang.org/p/MYDT8H_s6Z