Make sure history is saved after entering irb:rdbg sessions

IRB saves history when exiting `Irb#run` with `IRB.conf[:AT_EXIT]`. But
because `irb:rdbg` doesn't run inside `Irb#run`, history is not saved
when exiting. Therefore, we need to register the history-saving callback
manually to `at_exit` when we start `irb:rdbg`.

lib/irb/debug.rb

@@ -38,6 +38,10 @@ def setup(irb)
           irb.context.with_debugger = true
           thread = Thread.current
 
+          at_exit do
+            irb.context.io.save_history
+          end if irb.context.io.support_history_saving?
+
           DEBUGGER__.initialize_session{ IRB::Debug::UI.new(thread, irb) }
         end
 
@@ -48,6 +52,10 @@ def setup(irb)
           irb.context.with_debugger = true
           thread = Thread.current
 
+          at_exit do
+            irb.context.io.save_history
+          end if irb.context.io.support_history_saving?
+
           DEBUGGER__::SESSION.reset_ui(IRB::Debug::UI.new(thread, irb))
         end