encoding/xml: fix token decoder on early EOF

encoding/xml: fix token decoder on early EOF

The documentation for TokenReader suggests that implementations of the
interface may return a token and io.EOF together, indicating that it is
the last token in the stream. This is similar to io.Reader. However, if
you wrap such a TokenReader in a Decoder it complained about the EOF.
A test was added to ensure this behavior on Decoder's.

Change-Id: I9083c91d9626180d3bcf5c069a017050f3c7c4a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/130556
Run-TryBot: Sam Whited <sam@samwhited.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>