Fix setting method visibility on method wrapped with prepend

Ignore prepended modules when looking for already defined methods on a
class to set the visibility on.
[Fix GH-1834]

From: Dylan Thacker-Smith <Dylan.Smith@shopify.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/module/private_spec.rb

@@ -51,4 +51,43 @@ describe "Module#private" do
       Module.new.send(:private, :undefined)
     end.should raise_error(NameError)
   end
+
+  it "only makes the method private in the class it is called on" do
+    base = Class.new do
+      def wrapped
+        1
+      end
+    end
+
+    klass = Class.new(base) do
+      def wrapped
+        super + 1
+      end
+      private :wrapped
+    end
+
+    base.new.wrapped.should == 1
+    lambda do
+      klass.new.wrapped
+    end.should raise_error(NameError)
+  end
+
+  it "continues to allow a prepended module method to call super" do
+    wrapper = Module.new do
+      def wrapped
+        super + 1
+      end
+    end
+
+    klass = Class.new do
+      prepend wrapper
+
+      def wrapped
+        1
+      end
+      private :wrapped
+    end
+
+    klass.new.wrapped.should == 2
+  end
 end