Remove ModuleMethod#method_exists?

This is similar to the change in commit
8f58edd.

Commit e87c03b made
`ModuleMethod#method_exists?` redundant by changing the implementation
of `ClassMethod#hide_original_method` so that it uses
`ClassMethod#method_visibility` instead.

This introduced a subtle change in behaviour due to the difference in
`ModuleMethod#method_exists?` and `ClassMethod#method_visibility`. The
former would return false if the method being stubbed was on `Kernel` or
`Object` while the latter will return true in those cases. This gist
illustrates the difference -
https://gist.github.com/chrisroos/12a1b032b95664448c9e987132f33988.

Practically, this meant that _some_ stubbed module methods (those
defined on Kernel and Object) wouldn't have had the correct visibility
set. The tests added in this commit ensure we don't introduce
regressions in future. The tests for the visibility of the protected and
private methods failed prior to commit
e87c03b. I confirmed this by checking
our the commit before that (`git co
e87c03b~`) and running these new tests.

lib/mocha/module_method.rb

@@ -3,14 +3,6 @@
 module Mocha
 
   class ModuleMethod < ClassMethod
-
-    def method_exists?(method)
-      return true if stubbee.public_methods(false).include?(method)
-      return true if stubbee.protected_methods(false).include?(method)
-      return true if stubbee.private_methods(false).include?(method)
-      return false
-    end
-
   end
 
 end

test/acceptance/stub_instance_method_defined_on_kernel_module_test.rb

@@ -72,4 +72,67 @@ def my_instance_method
   ensure
     Kernel.module_eval { remove_method :my_instance_method }
   end
+
+  def test_should_stub_public_module_method_and_leave_it_unchanged_after_test
+    Kernel.module_eval do
+      def my_instance_method
+        :original_return_value
+      end
+      public :my_instance_method
+    end
+    mod = Module.new
+    assert_snapshot_unchanged(mod) do
+      test_result = run_as_test do
+        mod.stubs(:my_instance_method).returns(:new_return_value)
+        assert_method_visibility mod, :my_instance_method, :public
+        assert_equal :new_return_value, mod.my_instance_method
+      end
+      assert_passed(test_result)
+    end
+    assert_equal :original_return_value, mod.my_instance_method
+  ensure
+    Kernel.module_eval { remove_method :my_instance_method }
+  end
+
+  def test_should_stub_protected_module_method_and_leave_it_unchanged_after_test
+    Kernel.module_eval do
+      def my_instance_method
+        :original_return_value
+      end
+      protected :my_instance_method
+    end
+    mod = Module.new
+    assert_snapshot_unchanged(mod) do
+      test_result = run_as_test do
+        mod.stubs(:my_instance_method).returns(:new_return_value)
+        assert_method_visibility mod, :my_instance_method, :protected
+        assert_equal :new_return_value, mod.send(:my_instance_method)
+      end
+      assert_passed(test_result)
+    end
+    assert_equal :original_return_value, mod.send(:my_instance_method)
+  ensure
+    Kernel.module_eval { remove_method :my_instance_method }
+  end
+
+  def test_should_stub_private_module_method_and_leave_it_unchanged_after_test
+    Kernel.module_eval do
+      def my_instance_method
+        :original_return_value
+      end
+      private :my_instance_method
+    end
+    mod = Module.new
+    assert_snapshot_unchanged(mod) do
+      test_result = run_as_test do
+        mod.stubs(:my_instance_method).returns(:new_return_value)
+        assert_method_visibility mod, :my_instance_method, :private
+        assert_equal :new_return_value, mod.send(:my_instance_method)
+      end
+      assert_passed(test_result)
+    end
+    assert_equal :original_return_value, mod.send(:my_instance_method)
+  ensure
+    Kernel.module_eval { remove_method :my_instance_method }
+  end
 end