Add :stringify_names option to Psych.safe_dump

psych.patch

diff --git a/lib/psych/visitors/yaml_tree.rb b/lib/psych/visitors/yaml_tree.rb
index 5149178..673c8bc 100644
--- a/lib/psych/visitors/yaml_tree.rb
+++ b/lib/psych/visitors/yaml_tree.rb
@@ -70,6 +70,7 @@ module Psych
             fail(ArgumentError, "Invalid line_width #{@line_width}, must be non-negative or -1 for unlimited.")
           end
         end
+        @stringify_names = options[:stringify_names]
         @coders     = []
 
         @dispatch_cache = Hash.new do |h,klass|
@@ -328,7 +329,7 @@ module Psych
         if o.class == ::Hash
           register(o, @emitter.start_mapping(nil, nil, true, Psych::Nodes::Mapping::BLOCK))
           o.each do |k,v|
-            accept k
+            accept(@stringify_names && Symbol === k ? k.to_s : k)
             accept v
           end
           @emitter.end_mapping
@@ -341,7 +342,7 @@ module Psych
         register(o, @emitter.start_mapping(nil, '!set', false, Psych::Nodes::Mapping::BLOCK))
 
         o.each do |k,v|
-          accept k
+          accept(@stringify_names && Symbol === k ? k.to_s : k)
           accept v
         end
 

usage.txt

Psych.safe_dump({foo: { bar: "bar"} }, permitted_classes: [Symbol])
=> "---\n:foo:\n  :bar: bar\n"

Psych.safe_dump({foo: { bar: "bar"} }, stringify_names: true)
=> "---\nfoo:\n  bar: bar\n"

Psych.safe_dump({foo: { bar: "bar", narf: :crash} }, stringify_names: true)
[...]/psych/lib/psych/visitors/yaml_tree.rb:580:in `visit_Symbol': Tried to dump unspecified class: Symbol(:crash) (Psych::DisallowedClass)