[ruby/psych] Fix RestrictedYAMLTree allowing the Symbol class should allow all symbols

Ref: https://github.com/ruby/psych/pull/495

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

So `safe_dump` should mirror that.

https://github.com/ruby/psych/commit/592a75a656

1 files changed, 2 insertions, 2 deletions

diff --git a/ext/psych/lib/psych/visitors/yaml_tree.rb b/ext/psych/lib/psych/visitors/yaml_tree.rb
index 31858798e4..51491783c3 100644
--- a/ext/psych/lib/psych/visitors/yaml_tree.rb
+++ b/ext/psych/lib/psych/visitors/yaml_tree.rb
@@ -568,7 +568,7 @@ module Psych
           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
 
@@ -576,7 +576,7 @@ module Psych
       end
 
       def visit_Symbol sym
-        unless @permitted_symbols[sym]
+        unless @permitted_classes[Symbol] || @permitted_symbols[sym]
           raise DisallowedClass.new('dump', "Symbol(#{sym.inspect})")
         end