lua-yaml: fix strings literals encoding in yaml

yaml.encode() now wraps a string literal whose content is equal to a
null or a boolean value representation in YAML into single quotes. Those
literals are: 'false', 'no', 'true', 'yes', '~', 'null'.

Reverted the a2d7643c commit to use single quotes instead of multiline
encoding for 'true' and 'false' string literals.

Follows up #3476
Closes #3662
Closes #3583