Fix RestrictedYAMLTree allowing the Symbol class should allow all symbols

Ref: #495

That's how it works for `safe_load`:
```ruby
>> YAML.safe_load(':foo', permitted_classes: [Symbol])
=> :foo
```

So `safe_dump` should mirror that.

Author: byroot

lib/psych/visitors/yaml_tree.rb

@@ -568,15 +568,15 @@ def accept target
           raise BadAlias, "Tried to dump an aliased object"
         end
 
-        unless @permitted_classes[target.class]
+        unless Symbol === target || @permitted_classes[target.class]
           raise DisallowedClass.new('dump', target.class.name || target.class.inspect)
         end
 
         super
       end
 
       def visit_Symbol sym
-        unless @permitted_symbols[sym]
+        unless @permitted_classes[Symbol] || @permitted_symbols[sym]
           raise DisallowedClass.new('dump', "Symbol(#{sym.inspect})")
         end
 

test/psych/test_psych.rb

@@ -419,12 +419,15 @@ def test_safe_dump_extra_permitted_classes
   end
 
   def test_safe_dump_symbols
+    assert_equal Psych.dump(:foo), Psych.safe_dump(:foo, permitted_classes: [Symbol])
+    assert_equal Psych.dump(:foo), Psych.safe_dump(:foo, permitted_symbols: [:foo])
+
     error = assert_raise Psych::DisallowedClass do
-      Psych.safe_dump(:foo, permitted_classes: [Symbol])
+      Psych.safe_dump(:foo)
     end
     assert_equal "Tried to dump unspecified class: Symbol(:foo)", error.message
 
-    assert_match(/\A--- :foo\n(?:\.\.\.\n)?\z/, Psych.safe_dump(:foo, permitted_classes: [Symbol], permitted_symbols: [:foo]))
+    assert_match(/\A--- :foo\n(?:\.\.\.\n)?\z/, Psych.safe_dump(:foo, permitted_symbols: [:foo]))
   end
 
   def test_safe_dump_aliases