UMP workaround for browsers that implement only CORS http://codereview.appspot.com/4560046
UMP workaround for browsers that implement only CORS
http://codereview.appspot.com/4560046

From Kenton http://code.google.com/p/googleappengine/issues/detail?id=2994
CORS requires that browsers do an OPTIONS request before allowing
cross-site POSTs.  UMP does not require this, but no browser 
implements UMP at this time.  So, we reply to the OPTIONS request to trick
browsers into effectively implementing UMP.

R=metaweta@gmail.com


git-svn-id: eb21526166532bd3afc2c1499c0cf105ce815f5a@4487 360991c0-9f3b-0410-ba87-8b362a00edf4

initaldk/caja,jasvir/caja,initaldk/caja,cepm-nate/caja,googlearchive/caja,initaldk/caja,cepm-nate/caja,wishabi/caja-1,jasvir/caja,googlearchive/caja,wishabi/caja-1,jasvir/caja,wishabi/caja-1,jasvir/caja,cepm-nate/caja,googlearchive/caja,jasvir/caja,cepm-nate/caja,wishabi/caja-1,googlearchive/caja,googlearchive/caja,wishabi/caja-1,cepm-nate/caja,initaldk/caja,initaldk/caja,initaldk/caja,googlearchive/caja,wishabi/caja-1,cepm-nate/caja,jasvir/caja