Re-purpose wp_die() for ajax responses.

 * Allows unit testing of core ajax actions.
 * wp_die() now has separate filters to choose a handler depending on the context (ajax, XML-RPC, else).
 * wp_die) in ajax context does not need to be called with a string. Conversion takes place before die().
props kurtpayne, see #15327.



git-svn-id: https://core.svn.wordpress.org/trunk@19801 1a063a9b-81f0-0310-95a4-ce76da25c4cd