Add quotes for all values listed in https://yaml.org/type/bool.htmlA

Fix FasterXML#129

yaml/src/main/java/com/fasterxml/jackson/dataformat/yaml/YAMLGenerator.java

@@ -190,8 +190,10 @@ private Feature(boolean defaultState) {
      * better retain quoting for some values
      */
     private final static Set<String> MUST_QUOTE_VALUES = new HashSet<>(Arrays.asList(
+            "y", "Y", "n", "N",
+            "yes", "Yes", "YES", "no", "No", "NO",
             "true", "True", "TRUE", "false", "False", "FALSE",
-            "null", "Null", "NULL"
+            "on", "On", "ON", "off", "Off", "OFF"
     ));
 
     /*
@@ -955,11 +957,15 @@ private boolean _valueNeedsQuoting(String name) {
         switch (name.charAt(0)) { // caller ensures no empty String
         // First, reserved name starting chars:
         case 'f': // false
-        case 'n': // null
+        case 'o': // on/off
+        case 'n': // no
         case 't': // true
-        case 'F': // False/FALSE
-        case 'N': // Null/NULL
-        case 'T': // True/TRUE
+        case 'y': // yes
+        case 'F': // False
+        case 'O': // On/Off
+        case 'N': // No
+        case 'T': // True
+        case 'Y': // Yes
             return MUST_QUOTE_VALUES.contains(name);
         }
         return false;

yaml/src/test/java/com/fasterxml/jackson/dataformat/yaml/misc/ReservedValuesTest.java

@@ -0,0 +1,32 @@
+package com.fasterxml.jackson.dataformat.yaml.misc;
+
+import java.util.*;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.dataformat.yaml.ModuleTestBase;
+
+public class ReservedValuesTest extends ModuleTestBase
+{
+    private final ObjectMapper MAPPER = newObjectMapper();
+
+    public void testQuotingOfBooleanValues() throws Exception
+    {
+        for (String value : new String[] {
+                "true", "True",
+                "false", "False",
+                "yes", "no",
+                "y", "Y", "n", "N",
+                "on", "off",
+        }) {
+            _testQuotingOfBooleanValues(value);
+        }
+    }
+
+    private void _testQuotingOfBooleanValues(String key) throws Exception
+    {
+        final Map<String, Integer> input = Collections.singletonMap("key", key);
+        final String doc = trimDocMarker(MAPPER.writeValueAsString(input).trim());
+
+        assertEquals("key: \""+key+"\"", doc);
+    }
+}