Replace metaClass.getRealClass().isInstance(self) with method.getImplementationClass().isInstance(self) in Helpers.java so the protected-method visibility check inspects the method's implementation class and not the receiver's class.

Removes test_defined_protected_method exclusion from test/mri/excludes/TestDefined.rb.

Reproduction

class Foo
  def foo; end
  protected :foo

  def bar(f)
    defined?(f.foo)
  end
end
class Sub < Foo; end

p Foo.new.bar(Foo.new) # MRI: "method", JRuby master: "method"
p Foo.new.bar(Sub.new) # MRI: "method", JRuby master: nil ← bug
p Foo.new.bar(Class.new(Foo).new) # MRI: "method", JRuby master: nil ← same bug