Do not let the client cache future requests

Imagine we have a client whose clock is in the future. Then when it
makes a request in which the end timestamp is in the future and we
cache it, the next client who makes a request for the same timestamp
will get the cached version. But since the cached data is much older
there is a chance meanwhile there will be changes and we will serve
the wrong data.

liveblog.php

@@ -40,6 +40,7 @@ class WPCOM_Liveblog {
 
 	static $post_id = null;
 	static $entries = null;
+	static $do_not_cache_response = false;
 
 	function load() {
 		add_action( 'init', array( __CLASS__, 'init' ) );
@@ -92,6 +93,10 @@ function handle_entries_ajax_request() {
 			wp_safe_redirect( get_permalink() );
 		}
 
+		if ( $end_timestamp > $current_timestamp ) {
+			self::$do_not_cache_response = true;
+		}
+
 		$entries = self::$entries->get_between_timestamps( $start_timestamp, $end_timestamp );
 		if ( !$entries ) {
 			self::json_return( true, '', array( 'entries' => array(), 'current_timestamp' => $current_timestamp, 'latest_timestamp' => null ) );
@@ -377,6 +382,9 @@ function json_return( $success, $message, $data = array() ) {
 		) );
 
 		header( 'Content-Type: application/json' );
+		if ( self::$do_not_cache_response ) {
+			nocache_headers();
+		}
 		echo $return;
 		exit;
 	}