Add ignore_serialize for YAML::Serializable

[meatball133]

resolves #13345

Based on the pr of #11804, and should as that pr not include any breaking changes.

[beta-ziliani]

Thanks @meatball133! Overall it looks good, but the tests are failing. I haven't investigated the reasons, but it seems only the new tests are affected.

[meatball133]

It is one test case there should be one extra space. I did some tests with my new locally compiled Crystal, and it didn't have that space. But on the online test, it does. I will add the space in any case and hopefully shouldn't fail.

[meatball133]

Alright seems like different operating system wants different results.

[HertzDevil]

The trailing whitespace is tied to the libyaml version. It is fixed in 0.2.5: yaml/libyaml#186

Reproduction:

require "yaml"

class Foo
  include YAML::Serializable

  @[YAML::Field(emit_null: true)]
  getter x : String?

  def initialize
  end
end

# "---\nx: \n" before 0.2.5
Foo.new.to_yaml # => "---\nx:\n"

[meatball133]

So I don't understand, do I have to do something or will this issue automatically fix itself?

[HertzDevil]

The specs must either strip all whitespace at the end of each line or use different expectations depending on YAML.libyaml_version. There are a few examples of the latter in the spec suite