Visibility should begin from public for each scope

Even for singleton class definition such as `class << self` that
shares the same container with the outer scope, its visibility is
separated and set to `public` by default.

lib/rdoc/parser/ruby.rb

@@ -1782,6 +1782,7 @@ def parse_statements(container, single = NORMAL, current_method = nil,
 
     nest = 1
     save_visibility = container.visibility
+    container.visibility = :public unless current_method
 
     non_comment_seen = true
 

test/rdoc/test_rdoc_context.rb

@@ -928,7 +928,11 @@ def test_visibility_def
     assert_equal :protected, @c6.find_method_named('prot6').visibility
     assert_equal :public, @c6.find_method_named('pub6').visibility
     assert_equal :public, @c6.find_method_named('s_pub1').visibility
+    assert_equal :public, @c6.find_method_named('s_pub2').visibility
     assert_equal :public, @c6.find_method_named('s_pub3').visibility
+    assert_equal :public, @c6.find_method_named('s_pub4').visibility
+    assert_equal :private, @c6.find_method_named('s_priv1').visibility
+    assert_equal :protected, @c6.find_method_named('s_prot1').visibility
   end
 
   def util_visibilities

test/rdoc/xref_data.rb

@@ -75,6 +75,11 @@ def priv4() end
   public def pub5() end
   def priv5() end
   def self.s_pub1() end
+  class << self
+    def s_pub2() end
+    private
+    def s_priv1() end
+  end
 
   protected
   private def priv6() end
@@ -84,6 +89,11 @@ def prot5() end
   public def pub6() end
   def prot6() end
   def self.s_pub3() end
+  class << self
+    def s_pub4() end
+    protected
+    def s_prot1() end
+  end
 end
 
 class C7