Ensure that header elements do not impose artificial order when it's not necessary

[GoogleCodeExporter]

This was initially reported as changing the order of the Size and the
Length elements of a NeedleType.

All header elements should be examined to see if they need to impose order,
and if they do not, xs:all (or some variant) should be used to define them,
and the JiBX binding should be tested to ensure that it can accept
unordered elements.

Original issue reported on code.google.com by [email protected] on 2 Mar 2010 at 5:40