Add toJsObject method to Json

[guyonvarch]

Pull Request Checklist

• Have you read through the contributor guidelines?
• Have you signed the Typesafe CLA?
• Have you squashed your commits?
• Have you added copyright headers to new files?
• Have you checked that both Scala and Java APIs are updated?
• Have you updated the documentation for both Scala and Java sections?
• Have you added tests for any changed functionality?

Purpose

Convert values which type has OWrites typeclass to a JsObject with Json.toJsObject.

Use case

Convert a value to a JsObject and add a (key, value) pair to the generated JsObject.