Fixed EditChannelHeaderModal's contents not always being updated when a change is made

web/react/utils/client.jsx

@@ -590,7 +590,12 @@ export function updateChannel(channel, success, error) {
     track('api', 'api_channels_update');
 }
 
-export function updateChannelHeader(data, success, error) {
+export function updateChannelHeader(channelId, header, success, error) {
+    const data = {
+        channel_id: channelId,
+        channel_header: header
+    };
+
     $.ajax({
         url: '/api/v1/channels/update_header',
         dataType: 'json',