Add spec documenting the fact that null-ness doesn't persist between examples.

Author: myronmarston

Changelog.md

@@ -11,6 +11,9 @@ Breaking Changes for 3.0.0:
   leaving `double(name, methods)` for creating test doubles.  (Sam Phippen)
 * Remove `any_number_of_times` since `should_receive(:msg).any_number_of_times`
   is really a stub in a mock's clothing. (Sam Phippen)
+* Remove support for re-using the same null-object test double in multiple
+  examples.  Test doubles are designed to only live for one example.
+  (Myron Marston)
 
 ### 2.14.0 / 2013-07-06
 [full changelog](http://github.com/rspec/rspec-mocks/compare/v2.14.0.rc1...v2.14.0)

spec/rspec/mocks/null_object_mock_spec.rb

@@ -88,6 +88,17 @@ module Mocks
         # returns an int value from #to_int.
         expect(("%i" % @double)).to eq("0")
       end
+
+      it "does not allow null-ness to persist between examples" do
+        RSpec::Mocks.teardown
+
+        expect(@double).not_to be_null_object
+        expect { @double.some.long.message.chain }.to raise_error(RSpec::Mocks::MockExpectationError)
+
+        @double.as_null_object
+        expect(@double).to be_null_object
+        expect { @double.some.long.message.chain }.not_to raise_error
+      end
     end
 
     describe "#as_null_object" do