Round trip exception backtraces

This commit allows exception backtraces to round trip

ext/psych/lib/psych/visitors/to_ruby.rb

@@ -252,6 +252,8 @@ module Psych
 
           e = build_exception((resolve_class($1) || class_loader.exception),
                               h.delete('message'))
+
+          e.set_backtrace h.delete('backtrace') if h.key? 'backtrace'
           init_with(e, h, o)
 
         when '!set', 'tag:yaml.org,2002:set'

ext/psych/lib/psych/visitors/yaml_tree.rb

@@ -472,6 +472,9 @@ module Psych
           accept msg
         end
 
+        @emitter.scalar 'backtrace', nil, nil, true, false, Nodes::Scalar::ANY
+        accept o.backtrace
+
         dump_ivars o
 
         @emitter.end_mapping

test/psych/test_exception.rb

@@ -23,6 +23,20 @@ module Psych
       $VERBOSE = @orig_verbose
     end
 
+    def make_ex msg = 'oh no!'
+      begin
+        raise msg
+      rescue ::Exception => e
+        e
+      end
+    end
+
+    def test_backtrace
+      err     = make_ex
+      new_err = Psych.load(Psych.dump(err))
+      assert_equal err.backtrace, new_err.backtrace
+    end
+
     def test_naming_exception
       err     = String.xxx rescue $!
       new_err = Psych.load(Psych.dump(err))