encoding/xml: EncodeToken silently eats tokens with invalid type
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.