Fix encoding for explicitly marked str nodes

encode.go

@@ -447,11 +447,13 @@ func (e *encoder) node(node *Node, tail string) {
 				tag = ""
 			} else {
 				rtag, _ := resolve("", node.Value)
-				if rtag == stag {
+				if stag == strTag {
 					tag = ""
-				} else if stag == strTag {
+					if rtag != stag || isBase60Float(node.Value) || isOldBool(node.Value) {
+						forceQuoting = true
+					}
+				} else if stag == rtag {
 					tag = ""
-					forceQuoting = true
 				}
 			}
 		} else {

encode_test.go

@@ -494,6 +494,32 @@ var marshalTests = []struct {
 		},
 		"value: !!seq []\n",
 	},
+
+	// Enforced string nodes quoting with old-style booleans
+	{
+		&struct {
+			Value yaml.Node
+		}{
+			yaml.Node{
+				Kind:  yaml.ScalarNode,
+				Value: "y",
+				Tag:   "!!str",
+			},
+		},
+		"value: \"y\"\n",
+	},
+	{
+		&struct {
+			Value yaml.Node
+		}{
+			yaml.Node{
+				Kind:  yaml.ScalarNode,
+				Value: "yes",
+				Tag:   "!!str",
+			},
+		},
+		"value: \"yes\"\n",
+	},
 }
 
 func (s *S) TestMarshal(c *C) {