XMLRPC: Intoduce a date generation helper method to improve the dates returned...

XMLRPC: Intoduce a date generation helper method to improve the dates returned over XMLRPC when we have a 0 date stored for drafts.

This improves the ability of clients to work with the new wp Post APIs. See #18429 and #19733 props maxcutler. 


git-svn-id: https://core.svn.wordpress.org/trunk@20153 1a063a9b-81f0-0310-95a4-ce76da25c4cd