encoding/xml: Encoder duplicates namespace tags

[gopherbot]

by seanerussell:

== What does 'go version' print?

go version go1.2.1 darwin/amd64

== What steps reproduce the problem?

http://play.golang.org/p/3_oUruPYhq

== What happened?

Encoder.EncodeToken duplicates namespace attributes.

== What should have happened instead?

The encoded document should have had a single namespace attribute.

== Please provide any additional information below.

Attribute names on an element must be unique; this is a well-formedness constraint per
the XML 1.0 specification (http://www.w3.org/TR/xml/#uniqattspec). Per the
specification, both validating and non-validating parsers must report well-formedness
violations (http://www.w3.org/TR/xml/#sec-conformance).

Encoding and decoding XML documents should be idempotent and produce equivalent
documents.  This issue means that not only that decoding and encoding the result
produces a non-equivalent document, but that the document it generates is
not-well-formed.

This issue only occurs with namespaces.  Normal attributes are handled correctly.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-none.

[rogpeppe]

Comment 2:

Here's another example: http://play.golang.org/p/GTjuLNxE-d
This lack of encode/decode idempotency makes things awkward when trying to test for
expected output, as well as the lack of well formedness.

[gopherbot]

Comment 3:

CL https://golang.org/cl/179540043 mentions this issue.

[mikioh]

See #11841