Fix encoding for explicitly marked str nodes

[torkve]

When yaml.v3 encodes string nodes, that are explicitly marked with tag "!!str", it can remove this tag. But with ambiguous values like "y", "yes", "no" etc. it can lead to the result YAML document to be misinterpreted by YAML 1.1 compatible parsers.
This patch enforces old-boolean-like string nodes to be forcedly quoted.

[torkve]

Real world example: Google Gnostic compiler for OpenAPI encodes any single OpenAPI property name as YAML string, and if one have some Point-like model Point(x int, y int), after compilation it would become:

&yaml.Node{
	Kind: yamlv3.MappingNode,
	Content: []*yamlv3.Node{
		{Kind:  yamlv3.ScalarNode, Tag: "!!str", Value: "x"},
		{Kind:  yamlv3.ScalarNode, Tag: "!!int", Value: "2"},
		{Kind:  yamlv3.ScalarNode, Tag: "!!str", Value: "y"},
		{Kind:  yamlv3.ScalarNode, Tag: "!!int", Value: "3"},
	},
}

And after serialization it becomes

point:
  x: 2
  y: 3

Which would be deserialized incorrectly by YAML 1.1 compatible parser (e.g. one in kubernetes), since y would become true.

To the best of my knowledge I believe that Gnostic is correct here, since it explicitly marks node as string one and expects that it would always be a string in YAML, not boolean. That's why I propose this PR as a fix.