Fix yaml reserved words

[giga811]

yes, no are converted into true, false in yaml (until 1.1). (There was no documentation about this in 1.2)

Therefore these characters are converted into string "true", "false".

irb(main):022:0> "ኖᏃᓄᖺᘃのノ㋨ꆌ노".to_ascii
=> "falsefalsefalsefalsefalsefalsefalsefalsefalsefalse"
irb(main):027:0> '온'.to_ascii
=> "true"

I wrapped all the yes, no occurrences with brackets.

also created pr: https://github.com/rsl/stringex/pull/200