toString for schema

[georgolden]

• [x] tests and linter show no problems (npm t)
• [x] tests are added/updated for bug fixes and new features
• [x] code is properly formatted (npm run fmt)
• [x] description of changes is added in CHANGELOG.md
• [x] update .d.ts typings

[tshemsedinov]

Try toJSON method automatically calling by JSON.stringify

[georgolden]

landed in 1f7abf8ac0f10c7f4b9de9513138534b0cb931ec