Ignore unknown cast messages

This server needs to be bulletproof.

BugzID: 11762
diff --git a/src/rexi_server.erl b/src/rexi_server.erl
index 0f2e799..fff2558 100644
--- a/src/rexi_server.erl
+++ b/src/rexi_server.erl
@@ -74,7 +74,11 @@
         {noreply, St#st{workers = remove_worker(KeyRef, Workers)}};
     false ->
         {noreply, St}
-    end.
+    end;
+
+handle_cast(_, St) ->
+    error_logger:error_report({?MODULE, ignored_cast}),
+    {noreply, St}.
 
 handle_info({'DOWN', Ref, process, _, normal}, #st{workers=Workers} = St) ->
     {noreply, St#st{workers = remove_worker(Ref, Workers)}};