Generate attribute method unless it's already in the module.

There's no harm in generating a method name that's already defined on
the host class, since we're generating the attribute methods in a module
that gets included. In fact, this is desirable as it allows the host
class to call super.

activemodel/lib/active_model/attribute_methods.rb

@@ -329,7 +329,7 @@ module ActiveModel
 
       protected
         def instance_method_already_implemented?(method_name)
-          method_defined?(method_name)
+          generated_attribute_methods.method_defined?(method_name)
         end
 
       private

activemodel/test/cases/attribute_methods_test.rb

@@ -89,6 +89,29 @@ class AttributeMethodsTest < ActiveModel::TestCase
     assert_equal "value of foo", ModelWithAttributes.new.foo
   end
 
+  test '#define_attribute_method does not generate attribute method if already defined in attribute module' do
+    klass = Class.new(ModelWithAttributes)
+    klass.generated_attribute_methods.module_eval do
+      def foo
+        '<3'
+      end
+    end
+    klass.define_attribute_method(:foo)
+
+    assert_equal '<3', klass.new.foo
+  end
+
+  test '#define_attribute_method generates a method that is already defined on the host' do
+    klass = Class.new(ModelWithAttributes) do
+      def foo
+        super
+      end
+    end
+    klass.define_attribute_method(:foo)
+
+    assert_equal 'value of foo', klass.new.foo
+  end
+
   test '#define_attribute_method generates attribute method with invalid identifier characters' do
     ModelWithWeirdNamesAttributes.define_attribute_method(:'a?b')