Custom #to_json methods must accept the state parameter

[parkr]

This allows a Checkable to be a member of another item that has a to_json method, like Hash.

Example:

require 'json'
require 'github-pages-health-check'
entry = { check: GitHubPages::HealthCheck::Domain.new("www.parkermoore.de") }
entry.to_json

That currently fails with an ArgumentError, but this PR makes it work.