Add quotes to the strings "y" and "n"

'y' and 'n' are kind of ambiguous.  Syck treated y and n literals in
YAML documents as strings.  But this is not what the YAML 1.1 spec says.
YAML 1.1 says they should be treated as booleans.  When we're dumping
documents, we know it's a string, so adding quotes will eliminate the
"ambiguity" in the emitted document

Fixes #443

lib/psych/visitors/yaml_tree.rb

@@ -272,6 +272,8 @@ def visit_String o
           tag   = 'tag:yaml.org,2002:str'
           plain = false
           quote = false
+        elsif o == 'y' || o == 'n'
+          style = Nodes::Scalar::DOUBLE_QUOTED
         elsif @line_width && o.length > @line_width
           style = Nodes::Scalar::FOLDED
         elsif o =~ /^[^[:word:]][^"]*$/

test/psych/test_string.rb

@@ -17,6 +17,19 @@ def initialize
       end
     end
 
+    # 'y' and 'n' are kind of ambiguous.  Syck treated y and n literals in
+    # YAML documents as strings.  But this is not what the YAML 1.1 spec says.
+    # YAML 1.1 says they should be treated as booleans.  When we're dumping
+    # documents, we know it's a string, so adding quotes will eliminate the
+    # "ambiguity" in the emitted document
+    def test_y_is_quoted
+      assert_match(/"y"/, Psych.dump("y"))
+    end
+
+    def test_n_is_quoted
+      assert_match(/"n"/, Psych.dump("n"))
+    end
+
     def test_string_with_newline
       assert_equal "1\n2", Psych.load("--- ! '1\n\n  2'\n")
     end