fix: Allow the thread to be terminated (#81)

This was lost during a merge and causes the thread to not be
terminated when the application closes

src/Bugsnag/ThreadQueueDelivery.cs

@@ -60,7 +60,7 @@ public class ThreadQueueDelivery : IDelivery
     private ThreadQueueDelivery()
     {
       _queue = new BlockingQueue<IPayload>();
-      _worker = new Thread(new ThreadStart(ProcessQueue));
+      _worker = new Thread(new ThreadStart(ProcessQueue)) { IsBackground = true };
       _worker.Start();
     }