Pass headers in to endpoints

Note that the security of being able to override these isn't applicable, since
it's always set.

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2090 f69abe3c-2ee2-4550-9a6d-17bd458115be
Check If-Unmodified-Since on edit. Fixes #294

git-svn-id: http://gsoc.svn.wordpress.org/2013/rmccue/trunk@2091 f69abe3c-2ee2-4550-9a6d-17bd458115be