mGerrit: Fix ChangeList not loading / never updating

Its possible Request<T>#getHeaders() returns an AbstractMap so using put
on the map throws UnsupportedOpperationException and silently fails.

Then new bahavior is to add the AbstractMap to a HashMap which is
a supported opperation.

Change-Id: I42dfede6c8f06f413f1c1655650a3cf2facdbdef