Ensure stubs defined with allow syntax are cleared

spec/spectator/mocks/allow_spec.cr

@@ -9,5 +9,31 @@ Spectator.describe Spectator::Allow do
     it "applies a stub" do
       expect { alw.to(stub) }.to change { dbl.foo }.from(42).to(123)
     end
+
+    context "leak" do
+      class Thing
+        def foo
+          42
+        end
+      end
+
+      mock Thing
+
+      getter(thing : Thing) { mock(Thing) }
+
+      # Workaround type restrictions requiring a constant.
+      def fake
+        class_mock(Thing).cast(thing)
+      end
+
+      specify do
+        expect { allow(fake).to(stub) }.to change { fake.foo }.from(42).to(123)
+      end
+
+      # This example must be run after the previous (random order may break this).
+      it "clears the stub after the example completes" do
+        expect { fake.foo }.to eq(42)
+      end
+    end
   end
 end