Patch review requested: #9744 Adding DateTime#xmlschema

This patch fixes a nasty little bug that shows up when #xmlschema is
called on a Date/DateTime that's out of range of the Time class
(~1902-2038) -- you either get this exception:

SystemStackError: stack level too deep

or a fatal error, which brings down Mongrel or script/console, with
this message:

Illegal instruction

The fix is simple -- I've added a DateTime#xmlschema method, so that
Date#xmlschema doesn't get stuck in an endless loop of trying to
convert a Date to a Time.

http://dev.rubyonrails.org/ticket/9744

This patch fixes a nasty little bug that shows up when #xmlschema is
called on a Date/DateTime that's out of range of the Time class
(~1902-2038) -- you either get this exception:

Applied.