Split example from previous commit in two and reset Debugger

- there are two separate requirements, so each one gets its own example
- need to able to use the debugger when running rspec on itself

spec/rspec/core/configuration_spec.rb

@@ -514,12 +514,32 @@ def that_thing
     end
 
     describe "#debug=true" do
+      before do
+        if defined?(Debugger)
+          @orig_debugger = Debugger
+          Object.send(:remove_const, :Debugger)
+        else
+          @orig_debugger = nil
+        end
+        Object.const_set("Debugger", debugger)
+      end
+
+      after do
+        Object.send(:remove_const, :Debugger)
+        Object.const_set("Debugger", @orig_debugger) if @orig_debugger
+      end
+
+      let(:debugger) { double('Debugger').as_null_object }
+
       it "requires 'ruby-debug'" do
         config.should_receive(:require).with('ruby-debug')
-        Object.const_set("Debugger", debugger = mock("Debugger"))
+        config.debug = true
+      end
+
+      it "starts the debugger" do
+        config.stub(:require)
         debugger.should_receive(:start)
         config.debug = true
-        Object.send(:remove_const, :Debugger)
       end
     end