Make 'y' & 'n' into booleans (true/false)

[esotericpig]

Issue #443

In order to be compatible with Ruby's original Syck library, Psych made y/n default to strings. However, now, the Syck library is dead, and I think it makes since to make Psych compliant with YAML spec v1.1.

In the issue, this caused a problem when Psych produced a non-compliant file that then broke in a compliant parser in another language. I think it's important that Psych makes YAML files that can be consumed by anything that speaks YAML.

https://yaml.org/type/bool.html

However, this is a potentially breaking change. If previous gems relied on y/n as strings, they will now need to be updated to use single/double quotes. This is rare, but should be considered, especially for YAML common as this:

MySprite:
x: 100y: 100z: 1

It will need to be updated to this:

MySprite:
x: 100'y': 100z: 1

Maybe this should be a major version change? However, these gems relied on a broken/non-compliant parser, so it's a difficult decision to make.

Instead of this solution, we could...

• add some type of strict_compliance keyword arg option,
• or, output a deprecated warning to users for a couple of versions to update their YAML files.

[esotericpig]

The unsuccessful builds on Windows don't seem to be a problem from my code.

[tenderlove]

I think a strict mode would probably be good. We were talking about it in #426

I'm reluctant to change the behavior without asking people to "opt-in" because people are no doubt depending on this behavior. Even doing it on a major version bump is a tough sell. From a consumer's perspective they would think "I have to audit all of my YAML documents when I do this upgrade, but what do I get out of the upgrade?". Unless we can make the upgrade transparent (IOW users don't need to do anything), I'm not partial to a change like this as the default behavior.

[esotericpig]

A strict mode sounds good to me. I'll close this pull request.