Slight change to XMLDecl productions...

While I am still rather less than pleased by having the PI hack in the
XML spec (or rather, having encoding be part of it), I accept that the
probability of getting it removed is fairly small. So, in the interest
of interoperability, I would like to propose a very small change:

Currently "S" is defined with ideographic space as a component. As
ideographic space is *not* part of ASCII, I would recommend using a
production other than "S" as the separator in the XMLdecl; something
limited to just space and tabs (though it will be easy for authors in
Japan to make a mistake here, it's probably a reasonable tradeoff).