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EncodeToken takes a Token (i.e. an interface{}) as a parameter,
and expects a value of type StartElement, EndElement, CharData,
Comment, ProcInst, or Directive.

If a pointer is passed instead, or any type which does not match
this list, the token is silently ignored.

Added a default case in the type switch to issue a proper error
when the type is invalid.

The behavior could be later improved by allowing pointers to
token to be accepted as well, but not for go1.5.

Fixes #11719

Change-Id: Ifd13c1563450b474acf66d57669fdccba76c1949
Reviewed-on: https://go-review.googlesource.com/12252

Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>